GitLab Enterprise Edition offers a way to rebase the source branch of a merge request. This feature is part of the Fast-forward merge feature. It allows you to rebase the source branch of a merge request in order to perform a fast-forward merge.
You can configure this on a per-project basis by navigating to the project settings page and selecting the Rebase button checkbox. This checkbox is visible only if you have the Only fast-forward merging checkbox enabled.
Now, if a fast-forward merge requires a rebase, you will see a button to perform a rebase:
GitLab will attempt to rebase the source branch. If the rebase succeeds you will see an Accept merge request button. If a clean rebase is not possible, you will need to rebase manually. Rebasing may require some conflicts to be resolved manually.