Well, right now, bors doesn’t support GitLab at all. That’s going to need to be fixed before we’re going to be able to help you with setting it up at your workplace. @hauleth mentioned wanting this, too.
Also, it doesn’t support merging on other remotes. The way it currently works is to use GitHub’s API to perform the merge without downloading the source code, and for the hosted instance I don’t want to change that for perf reasons.