the game never once makes clear if Moriarty actually beats Gob. Other people might, but its never confirmed that Colin hits him.

The fact that he says, "Don't hit me." when you, the character walks in, suggests highly that it's the patrons that exercise the thrashing and not Colin.

I think you are being a little harsh on Moriarty. If Moriarty was in court today, the prosecution would have to prove that Colin beats Gob. But as it stands, Colin is innocent until proven guilty.
Sure, he's not nice to Gob. I understand that, but I don't think that he necessarily beats him.

If you can prove me wrong! That would be great! And I'm not being sarcastic. From what little knowledge and experience I got from writing about Moriarty, I couldn't tell you with 100 percent confidence that Colin doesn't beat Gob. But from what I have seen, and read, and looked at so far, the man is clean.

Post Script:

Put yourself in Gob's shoes. You went out for an adventure and get picked up by SLAVERS. Then, some half decent man buys you, gives you a job, a room, and food in a relatively safe community that has a pretty solid wall around it. Sure you have to repay the debt, and sure your boss is an ***, but really? I think the alternative could have been much, much worse. I believe its stated that its nearly 15 years of service Gob has to Colin. Most federal jobs now require 20 years of service to get some retirement check. In private companies its even longer.

After he repays his debt to Moriarty he can go out and seek his "mother". Or, more likely than not, since he has worked so long at the bar I'm sure Colin would be more than happy to keep him there working. Its not like Gob doesn't get paid either. The little that he does make, and from tips don't forget, could be stored away should he wish to venture out and return back to Underworld.