Initial create of local git repo...
git init
git add .
git commit -m "initial set of files"
Then you have gone to gitlab (other gits are available) and have created a new blank project. You can then get your local code into the remote repo using...
git push --set-upstream git@gitlab.example.com:namespace/nonexistent-project.git master
i.e. I did some work a short while back, got bored (distracted by shiny things), went away, and now need to pickup again!
Run the commands...
git checkout master
git pull
git checkout <feature-branch>
git rebase master
git push --force
This will update your local master to the latest and greatest changes in remote. And then move those commits over to your local repo feature branch, and then add your commits onto the end. And finally you will need to push (and force) to replace the remote history with this new rebased set of commits.
Worth being aware that this will blow away the remote branch and the set of commits it had before. So anyone who was looking at or using your feature branch will get confused (or broken).