Mmm, having not seen the water test results, I can't say for sure, but I'm starting to think that this is not a water quality issue. Especially when you say the bettas look fine, but the other fish don't, and it appears that you keep up the maintenance fine.
I think when you added the new fish to the tank you upset the aggression balance in the tank. I'm going to assume that the alpha fish was one of the bettas, and that when you added the new gourami to the tank it decided to re-assert it's alpha status and started beating up on all of the gourami's.
As for the pink gills, My guess is that when your gourami's got stressed they lost most of the color, and that caused their gills to look flesh colored.