the gasket consists of a white plastic part that fits over the 4 studs, and it allows the rubber boot to attach to it.
Do you need it? well it helps seal off the engine bay from the interior. it doesn't help maintain the seal on the vacuum booster so don't worry about that. the rubber boot just keeps things cleaner.
unfortunately this part isn't produced on the reproduction market and when you buy a re-manufactured booster they come with a generic boot installed that is different from original. functionally its the same but i noted sometimes the aftermarket boot doesn't fit in the firewall hole or can cause binding with the pedal linkage because its the wrong shape.
you can get away without it, the only to find one is from a donor car. however since all the donors are pushing 40 years old its very hard to find one in good shape.
its not super important, but its nice to have. sometimes the boot is so deteriorated that it can start to interfere with the pushrod operation and may jam the brake pedal, obviously in this case your better off without one.