Hi @manica,
The current link previews only work for certain specified websites, and there's currently no way to short-hand link to GitLab.
We plan to update the way link previews work, so that any webpage with open graph meta data will create a link preview. We may add special cases for websites without open graph data afterwards.
You can open a feature request on adding GitLab previews or short links to GitLab, or let us know if it's something you're interested in working on a pull request for.