I recall we considered this at various stages a few years back. I think this was around the same time we introduced our chatops integration.
I don't know the exact details and can't speak for others, but I suspect GitLab realised that chat(ops) tools are hard to sell and our time is better spent elsewhere. It's a shame though, having Gitter/chat directly integrated in a project (properly, not using e.g. an iframe) would've been pretty cool.
Ironically Gitter moving to Matrix probably makes this more likely - as integrating Matrix into GitLab would be much more sensible than integrating a single opinionated client like Gitter, given if GitLab properly spoke Matrix for chatops, it'd then be available to anyone in Matrix (Gitter, Element, or any platform bridged into Matrix - Slack, Discord, etc)
I don't know the exact details and can't speak for others, but I suspect GitLab realised that chat(ops) tools are hard to sell and our time is better spent elsewhere. It's a shame though, having Gitter/chat directly integrated in a project (properly, not using e.g. an iframe) would've been pretty cool.