Git is after all designed for this distributed scenario, so you'd think it would be possible, what about something like:

Setup:

    git add remote gitlab ...
    git add remote github ...

Then whenever you need to sync:

    git fetch --all
    git checkout -b gh --track github/master
    git checkout --track gitlab/master
    git merge/rebase gh
    git push gitlab

Could probably be done shorter but those steps should do it..