How to complete gang missions without killing everyone, and expanding your community instead.

Should work with any gang, except maybe for Billy's gang. Here is what you need to do:

1. Approach a gang location and save your game.
2. Go to one of the gang members without aiming your weapon (or he will attack you), and use the option "try it" when he asks for gold. If he starts attacking you, just reload and try another member.
3. Use "this is a shakedown" option and then "work for me".
4. Ask any other member to join you. If he refuses use shakedown and try again.

Note: this possibly only works when people have respect for you, so you will need to do some other missions first or try to use another person. Joe Wheeler usually works for me.