Honestly? I got them both side by side right now. I think I kinda wish Galaxy Screamer had finish of GT's Gorilla, and that's something I'd never thought I'd say about a GT product. To tap into my inner bobo skelly, Gorilla looks like a $20 upgrade in terms of paint and detailing. Like PE's Beast Gorrira, he just pops on his own minus the awful decision to not paint the eyes. I think Galaxy is great, but he has some niggles that frustrate me. Ratchets are too strong in the arms, wrists get cockblocked by the arm guards, accessories are lackluster, etc. These hold him back when he's otherwise dynamic and built like a tank.
In comparison, Gorilla joints are on point and the alt mode is a lot of fun if you can dig the look. That said, he's not perfect with the limited ankles and thigh swivels, and as people keep mentioning he does have a different plastic feel than anything GT has done before. He's heavy thanks to the diecast in the feet, but overall the plastic is both lighter and thinner in some areas. I don't think it's necessarily bad mind you, but it isn't the the same sort of feel you get from the brickhouse that was their Prime and Motormaster. It's a really hard thing to describe. If I had to compare it, some of the thinner plastic that peg on to the thigh area area feels more like Fansproject's Headmasters or their earlier Bruticus upgrade set. IE: Model kit plastic. Still he's left a much better impression than anything I've got from GT. If he had to throw down with something in his exact price range like Maketoy's Thunder Manus, I think he wins out.
I don't think I've had him long enough to give a verdict and avoid NEW TOY syndrome. I don't think you can go wrong with either though.