So suppose I have a Zoroark GX, and I want to put a Zoroark BREAK onto it. Can I do this legally?
No you can't do that. On the Zoroark BREAK card it says "Evolves from Zoroark" not "Evolves from Zoroark GX" which is regaded as a different Pokemon.
Source (I know this is about EX, but the same thing goes for GX)