Maybe it's because I'm familiar with almost all of the gems, but I don't see anything wrong with their Gemfile. It's a pretty complex project, and they really do have a ton of integrations and features that need those gems.
There's probably a few small libraries that they could have rewritten in a few files (never a few lines), but what's the point? The version is locked, and code can always be forked if they need to make changes (or contribute fixes).
You'd be surprised what you can do by carefully considering what the desired outcome actually needs to be.
Maybe there is justification for all the gems in gitlab's Gemfile, I didn't go through it with a fine tooth comb - but this reaffirms my experience that complex projects outgrow monolith codebases. Having an infrastructure outage take down your entire business is kind of a symptom of that.
There's probably a few small libraries that they could have rewritten in a few files (never a few lines), but what's the point? The version is locked, and code can always be forked if they need to make changes (or contribute fixes).