As far as I have been notice! GK rating does strongly depends on your defenders! It means, if you have a full 5 star defenders and they all nicely perform, this leads to a result of bad GK rating as your GK doesnt have much chance to show off his skills, he rather standing at once place sleeping or smoking or walking around seeing his defenders doing all the job! This is why if you have 5 star defenders, a 4 star GK is enough for your team, dont waste your tokens purchasing a 5 star GK or even a scout one if you have a good def line already! on the vice versa, if you have weak def line, 3 and 4 star defenders only, a 5 star GK will have big chance to get nice rating as he does most of the job! This is quite simple! Tell me what ya think bout it