Make sure you do not have any temporary branches (this means that you usually will only have two branches, main/master and develop)
Make sure you don't have any local changes.
Make sure you are on the main/master branch
git status
git checkout master
From your command line, make sure you have the latest code.
git pull --all
Mirror your repository to the new Github URL
git push --no-verify --mirror {github url}
Change your remote to the new Github URL
git remote set-url --push origin {github url}
Archive your old Gitlab account by going to Settings --> General --> Advanced --> Archive Project
Confirm your existing local copy is pointing to only the Github version
git remote -v
If it isn't, then remove the origins and re-add them
git remote rm origin
git remote add origin {github url}
git reset --hard