I want to run a script when a merge request that is generated from GitLab.
I 'think' I have 3 options:
I would appreciate some advice from anyone who has done this before and what would be the best method.
Git hooks are a native Git feature, git does not have a concept of merge requests - they are a feature of GitLab and competitor's platforms. As such a git hook will not meet your needs in this case.
GitLab webhooks are the solution you are looking for, as they can be triggered 'upon a merge request' being opened etc, see docs. They work by invoking a HTTP(S) endpoint, so if your script is set to run upon a request to this endpoint, or via some external build process (e.g. Jenkins) then these should meet your needs.
Triggers are for 'triggering' the rebuild of your project via GitLab (CI) (re-running a gitlab pipeline or external build), see docs. So they might meet your needs if the script to be run can be setup with gitlab CI or an external build process that can be integrated into GitLab.