I once heard someone describe this bar as, "Is that the one with all the stuff?". This is actually a decent interpretation of Golden Gate Tap Room. They have a pool table, a foosball table, and two of largest shuffleboard tables I've ever seeen (be warned the shuffleboard is $6 an hour instead of free, but it's actually worth it for the size). There's several arcade games and a ton of TVs. There's got to be like 30-40 beers on tap and even more microbrews in bottles. If you're having trouble picking they have a large wheel of beer you can spin to help make up your mind.
There's actually two entrances to the bar, which is on the second floor above a Walgreens. If you keep going up the stairs to the third floor you'll hit the restaurant portion of the place, which has a bar of its own. If you're having trouble at the main bar, know that you can sneek up to the third floor and get a drink quickly.
This bar isn't nearly as popular as similar bars, like the Wreck Room or Buckshot. I think the why is because it looks, and is, a bit of a tourist bar. Not only is it in Union Square, it decks itself out with a bunch of San Francisco gear on the walls, which always comes off as insecere. I know a lot of you have probably crossed this off your list on that basis, but that's one of the reasons I like going to this bar. There a tourists there, but it's nice to have a conversation with people who live outside of our bay area bubble, at least from time to time. And as I get older, I start to prefer toursists to the screaming 22 year olds you'll find and a similar bar.