You really do "commit a fix" but you commit it to your own git repository, then you make a "pull request" to the owner of the main repository, asking them to pull in your fix.
GitLab calles it a "Merge request", which is more fitting IMO.
EDIT: Admittedly if you are talking specifically of the request part, then you are correct. However the name comes directly from git's pull command which is a feature of git.
Merge-request makes more sense in Gitlab because they're main use-case is as a self-hosted internal tool where developers most-likely are pushing all of their feature branches to a central repository and asking for the branch to be merged into the mainline branch.
The command may exist within git, but it's not a part of most people's git workflows. "Pull Request" is a term that's far more commonly attributed-to and associated with tools like GitHub, Gitlab, Bitbucket etc.
Did you know that git has the ability to send patches via email, built in [1]? Git has a lot of functions that the majority of people don't use.
As another piece of evidence, see this page https://www.kernel.org/doc/Documentation/SubmittingPatches
It says Linus (and other kernel maintainer) will want a pull request from you if you want to contribute code, yet he thinks GitHubs version of pull request is, well, lacking (http://www.wired.com/2012/05/torvalds_github/)