I lived there for almost 17 years. It's an architecturally interesting and intriguing city but the gay life is almost non existent. Czechs can be closed and in some ways very snobbish. It takes time to get to know them. At the saunas (Labyrint is by far the best) you'll find guys hanging out together and not socializing. The guys moving around are usually other tourists and one or two Czech boys (rent boys usually). Another place you might want to try is a place called Heaven. It's a video store on Gorazdova Street. Upstairs you'll find videos, cock rings, dildos etc and downstairs is a rather dark cavern where guys go groping around. Every Wednesday night is naked night which is a lot of fun! Alcatraz has theirs on Sundays I think. Alcatraz is more hard core. You'll see fisting and fucking in the open - at least I did whenever I went.
Be very aware of pickpockets and NEVER EVER take a taxi off the street!! NEVER! Get the number for AAA taxi or Tic Tac Taxi and call them or SMS them for a pickup. Don't even let your hotel call a taxi. You'll be ripped off. I guarantee it!
Check you bill in touristy restaurants. They are notorious for adding things to bills including outrageous tips. If you do tip, 10% is standard practice.
If you go in the summer, go to Podoli swimming pool. There's a roof top tanning deck where you can get naked with about 100 other guys. You might even get lucky in the showers but there are people watching because of the kids.
And if you're up for a moonlight cruise, go to the Metronome which overlooks the city. You can't miss it! At night, in the park which surrounds the Metronome, are all sorts cruising around, having sex in the open etc. I've been fucked there quite a few times over the years.
If you have any questions, feel free to PM me. I hope I can be of some help.
I forgot to answer your question on where to live. If you're moving there, be sure you have a visa BEFORE you arrive. They are really cracking down on people trying to stay without a visa. Where to live, you'll want to check out Prague 3, Zizkov. It's a trendy, very gay friendly part of the city. It has direct access by tram and Metro (subway) to the center and has very nice clubs, restaurants, shopping and parks.
If you need a contact for finding a place to live, let me know. This is part of what I did while I was there. I'll connect you with good people who can help you get settled.