Hi getting_old
Firstly, I am really sorry to hear that you are going through such a worrying time, it must be so very difficult.
My partner is a dentist, and his brother was a maxillofacial surgeon, until he retired recently (slightly differrent to oral, I know). I spoke to my partner about your post, and he said "I'm really surprised that the GP said that, it must be a heck of a lump for him to be so sure". He also said that the most serious cancers tent to present as ulceration on the side of the tongue or the floor of the mouth.
So, as Taz says, nothing is definite yet, and the GP could very possibly be wrong.
Sending you hugs
Jeepers xx