Splitting this off from T265617: Experiment with GitLab-Phabricator integration.
There's a custom widget on our Phabricator instance that displays Gerrit patches associated to a Phab task with a Bug: trailer.
Here's an example:
@hashar discusses some of the implementation at T265617#7914463.
On the Phabricator side, this is handled by code in libext/misc/src/customfields/GerritPatchesCustomField.php.
I stubbed out a GitLabPatchesCustomField.php stubbed out, will push that somewhere.