The data will be kept on gitlab.
But mirrors can be added. (IIRC gitlab can even push updates to mirrors).
So a github (read only) mirror is likely.
Other mirrors (including self hosted) can be done, but I am not sure what the immediate plans are.
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Pull requests (git / not github) can be made for any public git server.
Even if you host your own (or use some other git hoster).
A pull request (git) is simply an email (or other form of communication) that says please pull branch FOO from my git @
http://server/reproAnd the person who receives the request then does
git pull http://server/repro FOO
Ideally that repro is a clone. Otherwise the pull retrieves the data, but the data is unrelated (i.e. no common ancestor)
git pull git@github.com:your-repo-ssh.git remote-branch-name