Currently gang crimes are selected by only one person and then the who gang participates because they are part of the gang. I agree that it seems silly since a gang crime should be committed by the gang, but as of right now as long as anyone in the gang clicks on a crime the whole gang gets credit for the crime.
Now, there will be a change soon to that that will make it so that only the leader can select the crime to be done and then everyone else logs in and assists in sub crimes that will allow us to accomplish the main crime, but that's not something that they've implemented quite yet, it's something they're working on implementing.
So...what does this mean? This means that we need to focus on keeping our gang crimes going at all times if possible. If you check out the
Gang Crimes in the wiki, you'll see that there are 3 crimes that give you 2,000+ gold per minute. Armed Robery would probably be the best, however we would always need people on every 5 minutes, and that would be a pain, so that's why Scuttle a Ship or Rob a Port would be the best as it gives you enough time to not have to worry about it with high yields. It's true that Slaughter Aztecs now gives you the most, but if you fail you've lost 12.5 hours instead of 4.
Hope this answered any questions you had, and if not, let us know!