When you're judging, you really don't have time to think about leo colors or whether you like the coach, since you're so busy keeping up with shorthand (and most judges have shorthand for specific deductions or problems in the skill/routine as well as the elements) and deductions. During the 3-day meets in particular (or long 2-day meets), you don't care what gym is going, as long as there is always someone on the equipment!
Judges can judge their home team, but they cannot be the head judge. I've judged my gym quite a few times, and it has always been fine. After one of the home meets, I was told that the parents were talking about my panel and how consistent we were throughout the day. Judging is a lot like gymnastics in that you get into a zone.
All of this. I've never judged (no attention span, could only end badly) but I've run/flashed/whatever next to judges who are judging a meet that the team from a gym they own or coach at is competing in, and I've been quite comfortable with how things appeared. There's going to be a problem if one judge is consistently scoring several tenths higher than the other judge and the situation wouldn't be set up where two judges from the same gym were judging together. The people who I have observed that are involved to that extent, to be owning, judging, coaching, they are very prominent in the gymnastics community, and pride themselves on their reputation. They aren't going to throw it all away to cheat on scores that wouldn't even mean anything. Yay, we won...but we were really worse. These people take gymnastics seriously.
It wouldn't even be in their best interest to do it anyway. I suppose it could happen unconciously, but I don't think so, because when you've seen a routine multiple times and know a girl, you tend to LOOK for what's wrong in it. i.e. you would know that you and the other coaches always have to get on this one about tightening her lower back - you would be more likely to look for the arch anyway and possibly perceive it to a greater degree than someone who you were impartial with. But even that, eh. I think most judges just keep moving and don't have time to dwell on it. Execution error, deduction...missing requirement, deduction. If there WERE an unconcious bias though, I would assume it to work against the girls the judge knows, not for them.