Player's ideal minutes are arguably between 48-80 minutes a game. If you intend to play a guy more than 80 minutes a game then pretty much expect a drop in GS.
Assuming you are saying you have 3 PO games, if you run your PG for 2 full games his minutes will be at 96 and yes his GS will be lower when he plays the third game.
You are right in saying that GS will reset at the end of the playoffs but you need your player to perform during the playoffs and not after.