Gang Hideouts show up on your map as a light blue marker, as a symbol of a fist. They show up regularly on your map. Scanning civilians and hacking them gets them to show up. If you haven't any on your map just carry on with the game/hack civilians, it shouldn't take long for them to show up by virtue of a message saying new gang hideout found. There are 15 in total.
Some tips: You must use 'B' to take down your target, you can't kill your target. You need to profile him. So even before you enter the hideout, profile everyone to find him. Simple advice but use cameras. This way you won't accidentally kill him. And also shows you where all guards are. Sometimes you can't find the target till you have made some noise and lured him out.
Disable any reinforcements on anyone who can call back-up. Nothing worse than being nearly done and having to deal with more enemies.
Use your environment well. There are plenty of opportunities to explode things for easy kills. Be sure your target isn't near an explosion though. You can use lifts, and other objects to cause distractions. And using cover is a given.
Use your devices to your advantage. If it's dark use blackouts to get close to enemies easily. Lure is also good if a guard won't come close. Jamming enemies headsets allows you to get close and take them down.
One thing that works very well in this game is using different entrances to the hideouts. If you can take a couple of guys down at one entrance do so and then exit and go around to another entrance. If you have been spotted or have been making noise the enemies will be focussed on where you was, leaving you to creep round to another entrance and take more people out whilst there attention is elsewhere. The guards never leave the hideout, but you can: remember that.
If you get the red marker showing someone calling the police then stop them! You don't want the police there making things difficult. You can aim your gun at them, grab their phone with 'B' or use jam comms. If the police do show, (and on a couple of missions I think it is unavoidable), then keep an eye on the helicopter and keep disabling its comms. Either hide till they are gone, kill them, or quickly finish the hideout.
Consider what's best for each hideout, sometimes you can be stealthy and take every guard out with your environment and by hand to hand takedowns, other times it is best to stay on the periphery of the location and take them out with a sniper rifle. On more than one hideout I literally found the guard, took him down, and left without killing anyone.
You have the option of killing all guards or escaping, so it's up to you, but escaping is safer imo. Remember where the exits are though.
Probably the trickiest hideout is the one where you have 3 guys to profile where the bridge is unfinished in Pawnee. Take the 3/4 guys out and drop through a hole in the ground. The police always came for me but once you are down there only 1 policeman ever made it down. Then get across, using elevators, stairs and containers.