I don't know what happened re: Djokovic/Gasquet as I have not been watching the match, let us for arguments sake say you are correct and indeed Djokovic tanked the second set.
If you were him and you were two breaks down at 4-0, would you bust a gut to get back in a set that you were likely going to lose instead of focus your energies on getting a solid start in the decider?
Think about it. If he was feeling tired, he therefore only has a certain amount of energy which he must conserve for opportune times, not blow his load in an attempt to lose a set 6-3 or 6-4 for the sake of some numpty who feels he should.
The players don't owe anything to anyone other than themselves, each player has to play to the way they feel is the best for them to give them the best chance to win. If Djokovic thought the set was gone and let it go, it's entirely up to him, the same for any other player.
He is not losing intentionally, he is letting something go which he feels is already lost in an attempt to focus on the larger battle ahead (ie. the deciding set).
Djokovic has just won another title, so he made the right decision, no?