https://drone.io is open-source and works very well.
I personally can't stand Gitlab's interface and general slowness.
sh.rt (which kind of became https://sourcehut.org) looks much better but it looks like you need to be logged in to see most of the content.
About abuse on Github, I only had issues with posting email addresses in public gists (even if they are public ones), and saw repo owners editing other people's comments, which should not be a thing.
About Github's source code, it's not open-source but you can host it with Github Enterprise and actually look at the source code (it's a very simple encryption of the source code as far as I remember).
Anyway, since Microsoft's acquisition of Github I started to worry of the general direction Github is going to but so far they haven't messed up too much IMHO.