You've picked three outfield players and compared them to a keeper
Also I can give you lots of top players who's had down years who's bounced back - doesn't mean there's a science behind it, it's football.
I don't understand what attributes DDG has that separates him from other keepers? He's still only 28 years old which is relatively young for a keeper and should be coming into his peak years. The issue with DDG this season has been his concentration not his athleticism regardless.
It could be possible that he's suffering from mental burnout or due to the fact that he's currently playing for a club who are currently on a downward spiral which could possibly affect the players. What Utd player has stood out this season after all? I could understand if Utd were playing well and he was making mistakes but his form is probably a direct consequence of how the club is performing.