You're right, it is totally unacceptable but totally unacceptable happens every day in every field of business so here we are. You really have to understand how logistics works these days. These parts are designed and manufactured all over the world and get shipped here, there, and everywhere. If GM looses a supplier, which is likely what happened, it takes time to make the deal, get manufacturing started, get parts shipped to warehouses, inventoried, and orders fulfilled. It's never intended to run out of parts. The 5th generation Camaro's release was delayed a few months because GM lost their supplier for some of the interior parts.
The OP hasn't stated (unless I missed it) whether he does or doesn't have a loaner or rental that GM is paying for.
Uhh, yes they do.
A few dealers do have the gasket but generally when a hot item like this is backordered, the dealers who have them have them "ordered for a customer" and won't let them go.