If you want to (or are okay with) searching all of the projects in a single GitLab (Enterprise?) instance, the existing search feature (as-of 2023-4-22) can do this.
If you're using a shared GitLab instance (e.g. gitlab.com
), right now, AFAIK, the only way to do this in the existing GitLab UI is to create a group, migrate or move all of the projects you want to search to that group, and then search just that group. (Yes, this is or can be a steep cost, and it's an annoying requirement.)
Otherwise, you'll need to find (e.g. this) or develop some kind of tool to use the GitLab search API for the projects you want to search and then combine the results somehow:
Search (REST) API: