I think two things happened.
1. He doesn't process the game at a high end level. I think he has the intelligence for a bottom four role, but his ability to read the game is pretty average for an NHL defenseman.
2. He never really developed physically. He is a beanpole and loses tons of physical battles. His skating is okay, but he's not really good at any skating aspect at the NHL level.
If Gormley was able to put on some weight and thus win more battles/be better against a forecheck I think he'd be a pretty good bottom four option, as he doesn't make a ton of mistakes and has some decent skill in transition.
Right now I think Gormley is a guy who really only makes sense as a bottom pairing guy/seventh defender if you have him matched with a physical defender next to him.
Suspect he would be quite good in Europe.