that is exactly how GSP should do it.
He is retired and not in any divisional contests. He is fighting interest fights and legacy fights only as he should, tied to big money. If there is a fight that motivates him to fight he should float it to the UFC and if they want to make, fine. And if not he stays retired. Simple as that.
The LW belt holds the right amount of reward against the risk he takes fighting again in a new division for the first time. Becoming MMA's first 3 division champ would be massive for his legacy so it makes sense for him. So now the question is does the UFC see it making sense for them and offer him that fight?