Just look at Google+: it still lives on for G-Suite customers while the public version has been shut down.
I don't expect GitLab to be of much interest to Google though: A lot of their stuff competes with services Google has on GCP, Ruby/Rails doesn't fit that well into their Python/Golang/Java stack and a remote-only company wouldn't fit into their office-bound culture.
Gitlab is big enough that should they ever be killed by a Google acquisition (I don't think that's likely), a strong enough community would sprout and keep maintaining it.
I'm not challenging necessarily, I legitimately am interested in examples.
OpenSolaris -> Illumos
Hudson -> Jenkins
OpenOffice -> LibreOffice
MySQL -> MariaDB (not killed, but forked out of fear it would be)
And a non-Oracle example:
Node.js -> IO.js (arguable - forked because Node dev was stagnant, eventually merged)
I admit it's difficult to come up with examples, though I don't know of any failures either. RethinkDB has been sort of a failure, but it was never very popular to begin with.
If you told me 10 years ago I’d be way more interested in working with Microsoft than Google I wouldn’t have believed you.