As Rick posted, you will have better luck at the Isuzu dealer for parts instead of looking at the Chevy dealer.
For gaskets, try Fel Pro HS8896S or Victor Reinz HS3706. The Victor Reinz gasket box that I have actually has the numbers HS8896S in small letters behind the part number, the same as Fel Pro. This is not surprising, as some gasket manufacturers will actually buy their gaskets from a competitor if the market is extremely small. (No, neither my FelPro gaskets nor my Victor Reinz gaskets are for sale)