What is the difference between the late model Porsche "GTB" and "GTC" engine ratings?

To the best of my knowledge (which is fairly extensive with most things "Porsche") there is no such thing as either a Porsche GTB or GTC. I have heard the GT3 Cup car referred to in some circles as a GTC as this is the POC name for the GT3 Cup Class; there is also a GTB class in POC racing and some of the older GT3 Cup cars should fit in here.

If that is what you are referring to, then the basic difference is that the GTC would be the current 997 based GT3 Cup car, while the GTB would be a range of specs of older 996 based models.

It seems most likely that several things may be crossed here:

There is an Americal Lemans racing class "GTC" in which Porsche has some cars, that are being referred to as GTC cars. These cars are Porsche GT3 (GT3 can look a lot like GTB, so I could see some confusion). If this is the case then the GT3 is the car that Porsche makes that runs in the GTC class as a GTC car. They are the one and same.

OR

911 GT2 = 911 race car that has a turbo.

911 GT3 = 911 race car that does not have a turbo.

(there are actually a myriad of other differences between the two, but that is the biggie)

Porsche has had GT2 and a GT3 factory produced race cars the public could buy. These have been based on model designations of 993, 996, and 997.

The 911 from 1993 to 1997 is a 993.

The 911 from 1998 to 2005 is a 996.

The 911 from 2005 to present is a 997.

GTB without being a typo of GT3 only comes to mind for me as either a Ferrari or an MG, but not a Porsche...

I have heard of the GT3, but not GTB or GTC?

I honestly have no idea.