Rules
Attacks have different value (low middle large), then there are 'other' type of attacks, such a sleeping attacks. They have their own values.

Attack Damage
low:
If the pokemon is strong to your attack, it will be 1 damage.
If the pokemon is normal (not weak or strong) to your attack, you will do 3 damage.
If the pokemon is weak to your attack, you will do 5 damage.

med:
If the pokemon is strong to your attack, it will be 3 damage.
If the pokemon is normal (not weak or strong) to your attack, you will do 5 damage.
If the pokemon is weak to your attack, you will do 8 damage.

big:
If the pokemon is strong to your attack, it will be 5 damage.
If the pokemon is normal (not weak or strong) to your attack, you will do 8 damage.
If the pokemon is weak to your attack, you will do 12 damage.

Battle
A GYM battle contains 3 pokemon of choice, the GYM leader must (by my personal rule) use 2 of the 3, as the pokemon type of her/his GYM. The other one is his/her choice. The challanger can use any Pokemon he or she would like.