It is a choke block off plate and I've not seen a gasket for it in any of my catalogs, and I have a lot of them. And
@Snoots is right, just get some gasket material, the heat resistant type and make a gasket for it. But that is if you know there's a leak coming from there. How's this, throw some soap on that intake and you might find any number of things that need tending to.
People, keep your engine's clean! You can keep it stock all you want and never touch it, but from time to time clean the engine of oil buildup, dirt, leaves, pine needles, dead bugs, and whatever else. Get a can of "Engine Bright" spray on engine cleaner, let it sit on a cold engine, agitate the heavy buildup areas, then after about fifteen or twenty minutes hose it off with a high pressure nozzle, like a regular water hose nozzle is enough. Take a zip lock bag and put it over the carb, if you want tie it on with a zip tie, don't spray directly on the distributor, and the rest will be okay. But keep it clean and you'll be able to spot those leaks long before you can't even identify what or where something is leaking or where it's leaking.
Yeah, I know I don't have any business telling people what to do, we're all adults. But you know, sometimes people have to be thrown a hint, a reminder from time to time, that's it's past due time to do some general house keeping.