Start Half-Life using "hl.exe -dev -console -game gearbox.
You may do this by modifying the windows shortcut properties for 
Half-Life OpFor and adding "-dev -console -game gearbox" to the 
end of the "Target" (or file to run). 

The following commands are available in console:

IMPULSE 101 - Give all weapons and ammo.
/GOD - god mode.
/NOCLIP - walk through walls/fly.
/MAP xxxx - go to map xxxx. 
/GIVE xxxx - give item xxxx.