Another can of worms Mrs What ;o) I guess you mean in the masonry close to the w/b .......... but not actually touching it.
I would say that ........ if it's a high kilowatt burner only a few inches from the wall, then ordinary gypsum plaster will shrink, craze, and possibly fall off.
On the other hand, if it's say, 12" (300mm) away from a moderate heat burner, then gypsum would probably be ok. It's all a matter of experience and on the spot judgement. I can't really be more helpful than that, I'm afraid.
Whereas a 25kg bag of skim would cost around £6, 25kg of Heat Resistant might be around £90........... I guess that's their problem.
Manufacturers are notoriously over-careful with their recommendations. This is where experience comes in.
To say that gypsum is as good as H/R is rather stupid though.
What are the clearances?