I feel like "gŵnwyr" (singular "gŵniwr") works better, although "gŵnyrs" would certainly be a possible loanword. It'd definitely be "gŵnio" though, I think that's the kind of conjugation you don't fuck with. I can't parse the last one, probably 'cause I don't know its English equivalent.
i don't actually speak any welsh, so that's helpful! the last one is from "goonstick"
Welsh likes to calque words like this, if it can. Stick would be "ffon", which probably keeps the same innuendo,1 though I'm not 100% certain. So "gŵnffon".
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I did look up a definition of goonstick to make sure I didn't go off course.