How to delete a commit that was already pushed to Gitlab

For the sake of simplicity, I’ll consider the case where you want to delete your latest commit. The command for this, locally, is git reset --soft HEAD~. If you want to delete older commits, you can use something like git rebase -i [commit ID right before the one you want to delete] (see more details there).

The problem I’ll focus on here is how to push your changes, now that you’ve removed something that was already pushed (and possibly added another commit on top). The command is trivial, it’s git push -f origin master (replace origin and master with the remote and branch names). But you may face a bunch of errors when trying to run it.

The first error I got was:

git@gitlab.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

I’m on Windows, and apparently Git is unable to use my SSH key, despite it being loaded in pageant (and as a matter of fact, usable when I push with SourceTree). The solution to this is to add the path to plink.exe in the GIT_SSH environment variable. You can set it permanently via the usual UI (or via Rapid Environment Editor), or you can set it temporarily, like I did, via the command line using set GIT_SSH=C:\PATH\TO\plink.exe. NB: don’t use quotes, or you’ll get something like error: cannot spawn "C:\PATH\TO\plink.exe": Invalid argument - error: cannot spawn "C:\PATH\TO\plink.exe": Permission denied. If you happen to have spaces in this path, maybe escaping them (slash-space “\ “) will work. Or just use the GUI. Or move plink to a folder without spaces.
If you’re on another OS, you might find a more appropriate fix for you in this thread.

The second error I got was remote: GitLab: You are not allowed to force push code to a protected branch on this project.
I had never protected a branch, but I quickly realized that Gitlab protects the master branch by default. You can unprotect it in Settings → Repository → Protected Branches (and once you’re done, maybe you’ll want to protect it back) (source).

And that’s all the trouble I had, git push -f origin master (or just git push -f if you just have one repo and one branch) should work now.