What would you like Renovate to be able to do?

The automergeStrategy configuration item introduced in #10627 should be implemented for the Gitlab platform, allowing users to configure the explicit merge strategy used by Renovate when automerging merge requests

Did you already have any implementation ideas?

The optional MergeStrategy argument can be provided to the GitLab mergePr implementation. The current implementation relies on GitLab using a "merge"-like strategy by default. The Renovate-specific strings for automergeStrategy=squash can be mapped to the parameter squash when submitting the accept-merge request to GitLab. It's not immediately clear if any other explicit strategies are supported by GitLab at time of writing.

If #10830 has been implemented, the arguments to mergePr may be different than what has been listed here. We should also remember to clean up the missing feature notice in lib/platform/gitlab/index.md and update the description in docs/usage/configuration-options.md if necessary.