I'm glad to hear you're getting a vet to look at this - certainly a troubling thing to see
Something like that would be hard to guess at by a photo. I've never seen such a thing on any of my own cats...I'm trying to think back to the clinic I worked in back in Ottawa...and the only thing that comes to mind is an abcess, but usually it's just swelling, not some discolouration - so in your cats case it would be black. The other thing that comes to mind - though I haven't seen one in a mouth that looks like that - is a tumour. Those are the only two things I can think of that I've seen to look even a bit like that. I've seen some ulcers on foal gums that look somewhat like that, but I'm not sure if those occur on cat mouths, or how they would come to be (the ulcers on a foals gums occur from too agressive bottle feeding, sucking on hard objects like the wall, or rubbing of the gums on a vinyl crash matt... )
Let us know what the vet says!