Minor complication - this gasket is no longer available. I tried ordering from that site and GM Parts Center, neither could get one. Guess I'm making one. Are these just a rubber sheet? 1/16" thickness probably?
RE this issue - The OE gasket is a low-resilience foam rubber. It is squished
totally flat between the brake booster and the firewall. It appears to be there only to do two things - First, to keep the brake booster from squeaking against the metal firewall while the vehicle is in motion. Second, to seal out air, dirt, moisture, and fumes from the engine bay so they can't enter the passenger compartment. The second purpose is far more important than the first.
That said, it appears you could use anything that is broadly resistant to heat and moisture and which will squish down between the booster and the firewall. A 1/8" thick piece of heat and flame resistant foam rubber with low resiliency would be perfect. However, if you want to take the easy way out, it's hard to see why a good metal-to-metal gasket maker with the appropriate resistances wouldn't also work fine. Just smear it on the firewall, circling the bolt holes and the center hole for the push rod assembly, bolt up the booster, and move on with your life. If you go this route, try not to put too much around the push rod hole as you don't want it to seep into the assembly on the booster.
Fabbing one up from a sheet of gasket material is much harder, thanks in large part to GM's fascination with oddly shaped holes. The hole for the push rod is shaped like an egg which makes copying it via simple measurement AND getting its spatial relationship to the bolt holes correct a difficult and time consuming task. Ask me how I know.
So to any future readers, if you want to try fabbing a gasket from a sheet of material, here's an idea - paint the surface of the firewall with a colorful, transferable substance which is easily cleaned off. Something like thin paint with a long dry time. Then, using a flat block at least as large as the whole gasket, press your gasket material sheet to the firewall to transfer the hole pattern to it, then cut out the holes. The outer dimensions of the gasket appear to be irrelevant - it does not fit inside of anything and there is plenty of spare room in all four directions, so err on the side of caution and cut the outside of the gasket larger than necessary. You can always make it smaller.
When replacing the booster, put the gasket on the booster so it is held in place, no need to try sticking it to the firewall, though if that works better for you go ahead. Torque bolts down, then shine a light through the footwell into the engine compartment. No light? Seal is good. Light coming through? Seal is bad. Simple.
