Ap Geiraduron puts the plural after the word-class/gender marker doesn’t it? If not, I’ve been reading it horribly wrong!
I see now that “gwrthrychafgwrthrych” does seem to be a run-on of two words, but it has been included (presumably in error?) as a head-word in the GPC app, and on its website:
Yes, usually it’s fine in Ap Geiriaduron. Now that I think about it, the problem tends to be when there is more than one plural form. They just get concatenated together.