PBR has the proper size white for Gulfstream, I've ordered several sets for several games including mine - measure hole to hole ... but you'll need to saw one side of the guide off to allow the rubber ring on the outer side facing the playfield to do its thing.

I'd use post studs/t nuts in replacement of the post screws that house the center plastics. Those posts get slammed constantly by the ball, and studs will hold up so much better. I've had them in my game for years now after farting around with loose screws and battered playfield wood.