Based upon your description the high loads are likely happening in the bolts in bending. There is some compression happening in the gasket but because it is likely a more compliant material than the metal parts, the stress is low. If it were a few hundred psi it would be blue by comparison to the 132,000 elsewhere.
In reality I would expect that you are applying a pre-load to the bolts sufficient to pre-stress the gasket. When the 300# downward load is added the compression in the gasket changes to counter-act the moment due to this load. I would also expect the downward shear load to be resisted by friction betwen the shelf base and the gasket.
My first approach would be to:
1/ bond the shelf to the gasket
2/ remove the screws from the analysis
3/ sketch circles to represent the contact surface of the bolt heads
4/ split each circle
5/ apply a load to the pseudo bolt heads representing the pre-load from each bolt
There is some interaction happening with the bolts taking some shear but I'd have to dwell on how to model that accurately.