I think that GK are hit and miss. You may get one that scores regular 7-8 ratings but it depends how successful your team is as a whole. If you’re winning most games your GK will generally have a decent rating especially with a clean sheet however if you lose the GK may only get 4-6 ratings.
I look for a tall GK when buying but that’s it as I rely on my front players to score the goals that win games. Occasionally though my GK will have an outstanding game making lots of saves and I reckon this is purely down to the game algorithms.