Necromunda "Rulebook" vs GorkaMorka "Da Roolz"
NOTE: The Necromunda Rules are still available from GW, and have recently been re-released. The GorkaMorka rules are sadly out of print.
Profiles
Both profile layouts are identical, as they are both 40k2 based. Orks have better stats than men, obviously. Diggas (GM humans) have normal Necromundian human stats.
Turn Sequence
Both identical.
Movement
Hiding is an optional extra rule in GM, in the "Clevver Stuff" section. Other than that tiny difference, the two rulesets are identical.
Shooting
GM has a few vehicle-related To Hit Modifiers (-1 if on a thrusting vehicle, -1 if target used thrusters).
GM has a different Injury Chart: 1-2 is a Flesh Wound rather than just 1. Also, Orks don't get pinned.
Overwatch is again an option extra rule in GM.
Hand-to-Hand Combat
Obviously models can't be Encumbered in GM, as they don't carry heavy weapons.
Parrying with swords in Necromunda is replaced by Blocking with Shields in GM.
Also, in GM there are no rules for Pinned Fighters in CC, Moving from CC or Breaking from CC (p29 in the original Necro Rulebook).
Leadership/Psychology
This is a big area of difference, mainly because Orks don't get scared easily! This means that in GM, all the psychology rules are Optional, from the "Clevver Stuff" section. There are also no rules for Keeping Your Nerve and Break Tests!
Bottle Tests are in the rules, inside Da Uvver Book's section on Scenario Special Rules.
Fear, Terror, Stupidity and Hatred are all identical.
GM Frenzy has an added rule for vehicle drivers- if a driver is frenzied, he will try to ram the enemy whenever possible! If an enemy model is within 6" of the front of his vehicle during its movement, he must attempt to ram them.
Necromunda's Advanced Game Rules
With the exception of the Psychological tests (see above) and Attacking Structural Features (see below), none of the Necromundian Advanced Rules are in GorkaMorka.
Attacking Strutural Features/ Da Fort
Nothing can't affect structures in GorkaMorka, except Gates of Forts. These count as stationary vehicles with AV 9, and can be rammed like a normal head-on crash. If you crash into it, the gate suffers D3 damage (unless you use a bike, in which case it suffers 1 damage). It has the damage table below:

D6	RESULT
1-2	The gate holds.
3	The gate is weakened. +1 for all further rolls on the chart.
4	A hole has been made! Only foot troops can fit through it though.
5	The gate is destroyed, leaving wreckage. Vehicles moving into the fort take 1 damage to the wheel/ track location.
6+	The gate is completely destroyed. It is now fully open.

Necromunda "Ash Wastes Vehicles" vs GorkaMorka "Da Roolz"
NOTE: "Ash Wastes Vehicles" are available as a .pdf download from GWs website. Click here for Ash Wastes Vehicles rules (from Issue 4). The GorkaMorka rules are sadly out of print.
Vehicle Types
In the GM basic rules, vehicles can be Bikes (Bike, Wheeled), Trakks (Light, Tracked), or Trukks/Buggies (Light, Wheeled).
Movement
Obviously you can totally ignore all the Necromunda rules for Walkers, Crawlers and Skimmers. Also, this is one of the areas where GM and Necromunda vary the most. You should actually ignore almost all the movement rules, except where noted here.
Vehicles in GM are all Fast vehicles, using special rules. Basically there are 4 types of movement. Gas Engines, Thrusters, Slow Speed Manoeuvres, and Tailing.
Chasing: Just like Necromunda Tailing, but as well as Tailing you can Race by getting 2" to the side of a vehicle. Also, note you both take Thrust Tests at the same time etc. And you can choose to give up the Chase at any time.
Gas Engines: This is the basic movement. Using these, you can go up to 3", then turn up to 45. Then another up to 3", and another 45 turn. Turns are from the vehicles midpoint. After this, you can choose to use Thrusters.
Slow Speed Manoeuvres: You can do this instead of using Gas Engines. You go 1", then turn up to 45. Then move 1", turn 45 again. Then a third time. That's the end of that vehicle's movement for this turn- you're not allowed to combine these with Thrusters!
Thrusters: These can be used after Gas Engines. You take a Thrust Test, and if you pass the vehicle can move up to 6". After that, you may try to do to a 45 turn, but you need to pass a Ld test. If you fail that Ld test, the vehicle goes D6" forward and then stops. If you don't loose control of the vehicle (or you choose not to turn), you can try another Thrust Test and so on. In theory, you can go forever!
Thrust Tests are fairly simple. For your first thrust on that vehicle in that turn, it is a 2+ on a D6. For your second, it is a 3+, and for your third or more it is a 4+. If you fail a Thrust Test, the vehicle may not move again in that turn. Also, you must roll on this chart:

D6	RESULT
1	Spin! Vehicle remains still, but faces a random direction (use the Scatter Dice).
2	The vehicle goes D6" straight forward, followed by a Spin!
3	Swerve! Turn 45 left/right (D6: 1-3=left, 4-6=right) and move D6" ahead.
4+	Move D6" forwards, and then stop.

Bikes and Trakks: Both move just like trukks, but bikes auto pass their first Thrust Test and Trakks only thrust 5" rather than 6".
Terrain: If any vehicles other than Trakks hit Difficult Terrain, they will Spin. Trakks treat it as normal ground. No vehicles can move in Very Difficult Terrain- if they hit it, they will spin and then be immobilized.
Transport
Transport Capacities are rather different in GM (at least in the official rules)- you can have as models on a vehicle as will fit! (See also Houze Roolz: Tranzport).
In GM, models can get on and off vehicles without difficulty before they thrust. Getting onto a thrusting vehicle counts as Boarding (see below). Getting off a thrusting vehicle is easy. Place the model beside the vehicle. Roll a D6, on a 4+ the model suffers a Str 3 hit. If they get hit they may not move further; if they don't they can feel free to move, run, charge, hide etc.
Vehicles Without Drivers
These simply swerve at the start of each movement phase (turn 45 left/right (D6: 1-3=left, 4-6=right) and move D6" ahead.)
Ram! Ram! Ram!
The Ash Wastes rules and GM rules are identical for all rams, collisions, crashes etc except for running people over.
When running models over, all GM vehicles (inc bikes) are Str 3 and bikes can't be hurt by the guys they run over.
Shooting
GM is similar to Necromunda, except where noted below.
Vehicles in GM are the only things allowed Heavy Weapons. They may also carry Linked Weapons (like normal weapons, but with a single Sustained Fire dice replacing their single shot). Bikes can't carry Heavy Weapons, only Linked.
If a vehicle's crew compartment is hit by a template weapon, every model onboard must make a seperate Armour Penetration and Damage Roll!
Boarding Actions and Vehicles in CC
Again, fairly similar except where noted below:
If you fail to board a vehicle it is a Str 3, not a Str 4, hit.
Official GM has no rules for "Hit and Run Attacks" (see also Houze Roolz: Hit and Run Attacks).
Vehicle Hit Location Tables
Trukks, Trakks and Buggies use the "Wheeled Vehicle Hit Location Table", with the following modifications:

D6	RESULT
1 Driver	As Necromunda, but on a roll a 2 there is no "Double 6" rule. If there is no driver, treat this as "6 Engine".
2 Fixed Weap	As Necromunda, but treat "Weapon Hit and Badly Damaged" as 4-5 rather than just 5.
3 Crew	As Necromunda, if there is no crew treat this as "1 Driver".
4 Wheel/Trakk	1-3: Upper Cog Links Damaged. Reduce all gas moves/each thrust by 1". Cumulative. 4: Drivel Pipe Bent. The vehicle swerves (45 turn left or right). 5: Transverse Knock Shells Damaged. The vehicle spins (random direction, use scatter dice). 6: Nob Pins Destroyed. The vehicle spins and is immobilised.
5 Gubbinz	If there is no gubbinz, the engines are hit. If the vehicle has any gubbinz, randomly select one. It is broken and thus unable to be used all battle on a roll of 4+ on a D6.
6 Engine	1: Thruster Jam- vehicle immediately thrusts in a straight line until you fail a thrust test or ram/crash! No lasting effects. 2: Gas Engines Damaged- they may not be used for the rest of the game. 3: Thrusters Damaged- they may not be used for the rest of the game. 4: Snapped Outright Strut- vehicle immobilised. 5: Gammy Shaft Broken- The vehicle spins and is immobilised. Roll a D6 at start of each subsequent turn. On a 6, explodes (see below). 6: Fuel Explodes- Vehicle immobilised. All warriors onboard hurled D6" in random directions, and suffer Str 4 hits. Vehicle Crippled.

Bikes use the table below:

D6	RESULT
1 Driver AV 8	If the driver is down, out of action or pinned the bike swerves, moves D6" forwards and falls over (immobilised). Anyone still onboard flies D3" and takes a Str 3 hit on 4+. 1: Kaboom! Take a Ld test, if failed you swerve. 2: Brake! Bike swerves. Warriors onboard roll under Str on a D6, or fall off (D3" away, taking a Str 3 hit on 4+). 3: Shrapnel! Str 3 hit on driver. 4+: Driver is hit by the weapon, or takes a Str 3 if it was a ram/crash/collision etc.
2 Fixed Weap AV 8	If there is no Fixed Weapon, count as a Wheel/Trakk hit. 1: Spark Tap Snaps! Weapon shoots wildly into air, spinning the bike. 2: Big End Jammed! May not fire next go. 3: Luck Nut Damaged! Weapon suffers -1 to hit for rest of game. 4: Crank Shaft Damaged! Must roll a 4+ on D6 before using the weapon. 5: Slush Clip Damaged! Weapon may not be used for rest of battle. 6: Weapon Explodes! Str 4 hit on driver, weapon useless for rest of battle.
3 Crew AV0!	Randomise which crew member. Any warrior down or out will fall off (lands D3" away, Str 3 hit on 4+). 1-2: Stray Shot! Crew leap to one side, the bike swerves. 3-4: Blast! Must roll under Str on a D6 to stay onboard. If fails, fall off (D3" away, Str 3 hit on 4+). 5: Shrapnel! S3 hit. 6: Hit by weapon as normal, or suffers Str 3 if it was a ram/crash/collision etc.
4 Wheel/Trakk AV 8	1: Intermediate Cog Links Damaged. Reduce all gas moves/each thrust by 2". Cumulative. 2-3: Drivel Pipe Bent. The vehicle swerves (45 turn left or right). 4-5: Flange Bearing Damaged. The vehicle spins (random direction, use scatter dice). 6: Nob Pins Destroyed. The vehicle spins and is immobilised.
5 Gubbinz AV 8	If there is no gubbinz, the engines are hit. If the vehicle has any gubbinz, randomly select one. It is broken and thus unable to be used all battle on a roll of 4+ on a D6.
6 Engine AV 10	1: Thruster Jam- vehicle immediately thrusts in a straight line until you fail a thrust test or ram/crash! No lasting effects. 2: Gas Engines Damaged- they may not be used for the rest of the game. 3: Thrusters Damaged- they may not be used for the rest of the game. 4: Snapped Gork Strut- vehicle immobilised. 5: Gammy Shaft Broken- The vehicle spins and is immobilised. Roll a D6 at start of each subsequent turn. On a 6, explodes (see below). 6: Fuel Explodes- Vehicle immobilised. All warriors onboard hurled D6" in random directions, and suffer Str 4 hits. Vehicle Crippled.

Vehicle Costs
In Official GM; Trukks/Buggies cost 20 Teef, Trakks cost 15 Teef and Bikes cost 10 Teef (see also Houze Roolz: Tranzport).

Ork Mobs in Campaigns
I don't have time to write up *all* the rules for ork mobs in campaigns, and it probably wouldn't be legal! Try to get a copy of the GorkaMorka "Da Uvver Book". It contains all the GM fluff, plus the mob list, all the scenarios, campaign rules, etc.