Convenience costs. Want it fast? Pay the man. That's why many times you stop at a "gas n go" place (name your favorite brand convenience store) and pay $2 for that bottle of Iced Tea or whatever when it's $1.59 anywhere else. But you're in and out in 2 minutes and your car doesn't depreciate while it's sitting in the parking lot. Pay extra to UPS/USPS/etc., and they'll bust their butts to get it to you tomorrow. WTF? So you have to pay more for someone to do their job more efficiently, but yet the person actually doing it doesn't get paid any more money...hmm. And if you don't pay extra, USPS has already said they're going to go even slower on First Class deliveries and such. If you're not in a hurry, that's a plus for you.
Speaking of that, even if you are, more money doesn't get it to you faster. Here's a beauty- some places are so backed up with backorders, you have to wait almost a year. I think Legendary Interiors has about a 275 day waiting period for interior panel orders right now. Used to be a couple of weeks.
But I digress. Yes, there is a gasket behind the splash shield. GM p/n 3966202 superseded to 15699959. It's a thin black foam-like gasket. Seals out the dust and junk where the backing plate meets the steering knuckle. I do not know if there is an aftermarket equivalent.
I knew I should have stocked up on those GM splash shields when I had the chance. They used to be pretty cheap. I paid less than $20 per pair over the counter around 1993. Only got 3 sets left.
Splash shield junk fits pretty much all A/G-body disc brake applications 78-88. If worse comes to worse take a drill and hacksaw to an old aluminum pie plate.