Is there such a thing?? Don't recall seeing any and would think that such a gasket might push the wastegate puck away from the turbo's hole enough so that the puck cannot fully seal the hole.
Somebody makes a downpipe to turbo gasket but I forget who, it's really not needed there tho, you can use stuff like the orange hi-temp silicone if you really want a gasket seal there.
The others I saw were different? Look around some.
PS: Personally I wouldnt use one. Just make sure its all flat & then either some Axle Grease (heat turns it to a Carbon like gasket) or the silicone in a Small smear.