Hope Rocco is fine and all healed now, Susan
One of our boys, Coco, had the same thing removed today. I found it at the front of his mouth last Friday and like you, was mortified it hadn't been spotted. It was right at the top of the gum above his teeth and very difficult to see. We saw the Vet that day and she explained epulis to us, benign, but that it had to be removed esp. as he could knock it and have a bleed.
They took the opportunity to do a dental at the same time, which was about due. He is "resting" in his furry igloo as I speak, he is a bit of a wimp anyway, but sleeping it off is best. Our other two (Rascal and Bandit) are full of concern, so sweet.
The vet said it may grow again, may not.
I found this
Epulis - Wikipedia, the free encyclopedia