If Google acquired GitLab, they'd offer it as a managed platform for GCP, which they're unlikely to shut down since contracts and SLA require a heads up of about a year before they can remove a service that is GA from their platform.

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.