Yes, I think the Gamma Corrector really only gives you LUTs. If there was interpolation the bit depth would change.
What do you mean by add/subtract from gamma? I'm afraid I don't quite follow. Do you mean changing the gamma value? That is, of course, always a non linear operation. Is that what you meant?
Ok, so we have established that the gamma corrector is not to be used for applying gamma or removing it, thus converting from and to linear space. Or at least not for high quality applications. Doing a decompression should result in a higher bit depth, and compressing reduces the depth.