Gang missions - are sorted by members needed to start the mission - Name of the mission has nothing to do with anything its just a name.

So if you have 10 members in your gang - starting a 20 man mission just will not work as you are 10 members short

Start the lowest mission you can which is 2man missions, work up from there

As for what they give,

you get battle points (which is used in gang)

Cash 

Also give this a look https://docs.google.com/document/d/1654JNmZ_9ZaijG9cVc2CG71DdjZvTXhBCcuWWSe2tFo/edit