IIRC Gogs/Gitea and GitLab have plans to enable pull request federation, so you could use your own instance to make pull requests on a completely different one.
I also recommend to check out Gitea (community fork of Gogs) if you haven't yet, it's a nice alternative to Gitlab for selfhosting.
I also recommend to check out Gitea (community fork of Gogs) if you haven't yet, it's a nice alternative to Gitlab for selfhosting.