I have bought grips from Gun Parts Corp. for around a dozen guns to replace hard rubber grips. The new ones are plastic, of course, but all that I have used were excellent copies and just about indistinguishable from the original. In fact, I have some guns with the original grip on one side and the repro on the other, and there is no difference I can see between them.
AFAIK, no one makes or sells new gutta percha grips; for one thing, the natural product is not even harvested any more and if it were no one would use it since modern plastic is so much better. Some people wonder why gutta percha was used at all, but when those grips were new they were fairly pliable and tough; the substance hardens and becomes brittle with age.
I will add one more note. I have put modern J frame wood grips on some old .38 breaktop S&W revolvers. Incredible as it is, the critical distance from the inner curve of the frame to the grip pin did not change in nearly 70 years, even though about 1/4 inch was added to the bottom of the grip. A fine match is possible just by trimming the J frame grips to match the breaktop frame. They don't look original, of course, but they look great.
Jim