The keepers are difficult to rate, because the difference between a good keeper and an excellent one is in fact quite little. Here is what i learned about Goalkeepers:
Defence stat stands for their positionning in the goals facing a shot(how often and how well they are on the target to stop a shot, the quality of their steps when facing a striker who is going to shot.
GkS stat stands for the quality of the gk's handling(his catching of ball, for instance Landreau is very average at it, because he often punched the ball instead of catching it and sometimes make big handling mistakes).
As for Costil he's doing a very good start of the season, but he hasn't played enough in first division to be considered better as many ligue 1 keepers, i think.
You already decreased some of his stats, but i would also decrease his RES to RES:88.
I' ll try to watch him play more often to have a better idea of his abilities.