Air Force Platform One uses it.
It's hyperbole to say you can use it for the entire lifecycle. That might work fine for one-product monorepo shops, but Gitlab still doesn't scale well right now. Aside from basic issues like the runners not working well, the binary artifact repositories are way behind what something like Artifactory or Nexus offers. It's extremely annoying not having group and server scoped registry tokens except for the container registries. It makes it more difficult than it should be to publish modular libraries to be used elsewhere in your organization by other products. It's effectively unusable if you're working behind an air gap and trying to mirror public registries. The only way I can think to do it is creating a dummy projects with every kind of package registry enabled and push all of them into that one repo, but that won't work for things like Maven that have a notion of namespacing.
It can't really replace something like Jira, either (and in Platform One's case, it doesn't). You can only create an issue in a repo, but there is plenty of work organizations do and want to track and organize that can't be directly tied to a code change, let alone a code change in only one repo. So where do you raise an issue to track work not related to writing code or related to writing code but across multiple repos? I've seen people create dummy repos that serve no purpose except being a central place to put issues, but that is just working around the limitations. It's fine as a bug tracker, but not a general purpose work tracker and project management solution.