This song helps! :P Gwena
It sure does :) Diolch!
Is it strictly a verb, or can it also mean "smile" as a noun?
No, "Gwên" is the noun.
This goes into my collection of words that result in the speaker smiling due to shaping the mouth as they're spoken. It joins the ranks of such phrases as "say cheese" in English, or "茄子" in Chinese.
True though remember it'll only do this in the southern dialects where the sound is "ee".