Here's the short version...

Gang Funds

100 paragraphs gets the gang $250. 15 paragraph discount per additional member, up to 3 additional people. The gang gets $25 per person in each quest or duel in the gang.

Gang XP

Do RP related to the gang, get XP for the gang. More XP means more rep.

Turf

Do RP to get and keep your turf. Don't keep up the RP, you'll lose the turf. Turf doesn't give you anything special except for a more safe zone for your gang to be in.

Buildings

100 paragraphs gets you a building. 50 more paragraphs gets you security. This paragraph requirement can be divided up amongst the gang members however you want.

Non-gang members can use these rules for official buildings.

Gang Wars

Fight to your heart's content. No special rules or restrictions on gang wars, kill who you want.

Multiple Membership

You're allowed to be in 5 gangs max.

NPC's

100 paragraphs to get a level 1 NPC for the gang, and additional RP to level the NPC up.

Non-gang members can use these rules for official NPCs.

For more details and reasonings, please read the longer version of the rules.