Ok this might be completely off topic, but I'm curious about something.
Since I started playing e5 agaisnt e4 not too long ago, I obviously face the King's gambit quite a few times.
What I would like to know is, if it's common for black to castle queen side? because that's what I aim for a lot of times.

While I agree that 3.Bc4 may be a normal move for class players, I should warn that it's regularly played for precisely the wrong reason. Most class players think that 3.Bc4 prevents the immediate 3...d5, but factly 3.Bc4 INVITES 3...d5 to be played to good effect!