[old Chevy guy]

I have been working on Chevy cars and trucks since I was 16 and now I'm......well for a long time. I've never once seen a gasket like that behind a brake drum. The grease seal keeps dirt out and grease in.I totally understand you want it to be 100% but I don't think any inspector is even going to know that there's supposed to be a gasket in there at all. If you can get it no problem, if not don't sweat it.