Hi,
Maybe it's just the angle of the photo, but your GBR (I believe that's a male by the way) look to have a strange body shape.
I suspect these are "balloon GBR", which is essentially a GBR bread to exhibit a shorter, rounder body shape. They are intrinsically weaker than traditional GBR (which are a fairly weak and short lived fish at the best of times).
It therefore doesn't surprise me to hear of them become sick (sorry).
The illness you describe sounds like Gill Flukes / Gill Mites, but I see no sign of that on the fish. The white marks / bobbles look like potential turmor or Lymphocystis, neither of which are treatable. On the plus side, neither kill fish very quickly if at all and these diseases are not transmittable to your other fish.
Sorry I couldn't have been of more help.