Gk rating high-> weak def line and your GK has a many chances to do his job
GK rating low-> strong def line and your GK has nothing to do during the match.
When your gk makes a fine save, his rating is higher but only mean higher, doesnt mean if your gk make 2,3 fine saves in 1 match, he got rating of 9 by the end of the match.
This is why, GK is the most useless position in top 11, so stick with your 3,4 star GK if he is still contributing, you dont have to waste token for a 5 star one.