How is this a "true or not true"-question?
KG has stated publicly that he will take it year to year with his current contract and has hinted at retirement several times in the past. However if he still feels the game after this season and believes the Celtics can make a deep playoff run next season I'm pretty sure he'll be back.
I don't think there's a real answer right now. Too much is depending on what happens for the rest of this season...