We support searching GitHub topics with repo:has.topic(mytopic). We do this by ingesting topics when we sync the repo, then we have a database index on that field in the metadata column that allows us to do quick filters against the topics.

In order to support topic search for GitLab, we'll have to make sure that we actually fetch topic data during syncing (I don't think we do right now), then add a DB index on that field in the metadata so we can search over it efficiently. We might want to reach out to the Source team about syncing the topics.