Ah! Yes. No reason why you couldn't. It would require making a new repo, copying the files into the new repo, and committing in one big commit before pushing to gitlab, but yeah. Definitely doable.

(I basically always do this myself. I don't start the Git repo until I want to Open Source it. So when I first Open Source it, it's a "complete" (or at least "minimum-viable-product") project and there's only one commit. Every commit I make and push thereafter is public, but there aren't any from before my first push/publish.)