I posted a few short replies from my phone last night, but now I'm in front of my PC and wanted to elaborate a bit.
It's my understanding that the first Predators (or POTO Funk Master Kraks) came in to the country on a rock with GMK. So, the assumption would be that it was just a color morph of GMK.
That being said, I have both Predators and GMK in my tank with the same nutrient levels, same light schedule and even under the same PAR (within 10-15%). My GMKs still look like GMKs, and my Predators still look like Predators. My GMK is up to 16p (from a single polyp), all with the same pattern. My Predators are up to about 8-9p (from 2p), and they all have the same pattern.
So, if it is just a morph, they are holding the colors and pattern well. I don't see any signs of either of them "changing back".