From Gitlab UI you can not.
Actually, from GitLab 15.3 (August 2022), you can:
Rebase a merge request from the UI without triggering a pipeline
In large and busy monorepos with semi-linear branching, you might need to rebase your merge requests frequently. To save resources, you might not want to run a pipeline each time you rebase. You could skip the pipeline while rebasing with the API, or by using Git push options or [ci skip]
in a commit message, but not when rebasing from the UI in a merge request.
Now we have an option to skip the pipeline when rebasing from the UI, so you have better control over when a pipeline runs for your merge requests. Thanks to Kev for the contribution!
See Documentation and Issue.
To rebase a merge request’s branch without triggering a CI/CD pipeline, select Rebase without pipeline
from the merge request reports section.
This option is available when fast-forward merge is not possible but a conflict-free rebase is possible.
Rebasing without a CI/CD pipeline saves resources in projects with a semi-linear workflow that requires frequent rebase