By the longest of long shots, yesterday's guest speaker at the Healthy Beginnings drop-in was a dental hygienist. So I made her look in Gwen's mouth to tell me what the weird white thing was.
To my IMMENSE relief, she did not tell me it was thrush.
No, it's an Epstein pearl. I had previously come across this term in my tooth-denying research, but discarded it as an option because every description I found said they were found on the roof of the mouth, not on the gums. But the hygienist said they could be anywhere in the mouth. And she did say it looked a whole damn lot like a tooth, to make us all feel better about being so very misled.
Short story, if you don't want to follow the link: painless, symptom-less, not to be worried about, and will go away on its own. My favourite weird Gwen glitch so far.