Do you think many users will mistakenly believe Gitlip is an official Git project put out by the same authors as Git?

There can't be trademark infringement unless there is a likelihood of confusion.

I don't know. I do know that an incredible number of users do not understand that GitHub is NOT an official anything, and so I feel like we have an existence proof of this being a serious concern. (That said, I also can see an argument that once git allowed that to happen without a fight, or even AFAIK at least some retroactive agreement--and then further also allowed GitLab--that they don't even have an enforceable trademark anymore, at least with respect to this kind of prefix use. But like, this is a kind of oblique argument that I don't think you are already making.)