This might already exist but I couldn't determine how to do it.
Could you add a feature to the challenge checker searching so that users can filter the checkers list by GC code. If I know a particular challenge I think it would be useful to search for that GC code to see if a checker already exists. This might eliminate a lot of duplicate questions asking for a checker too.
I'm assuming this would be fairly trivial to add as the challenge code is clearly tagged against a checker. Similarly being able to see the list of challenges a particular checker solves would be handy. As users could see which ones they'd completed and then view that on a map.