NAPA sells gasket material. You can punch/chisel a gasket out of this gasket stock

Or, buy an A14 manifold gasket, then chisel the ports bigger before installing. This works if you find a gasket without metal-rimmed intake ports -- and one that goes above the ports far enough to enlarge to the GX size

Or, get the Nismo/Nissan Competition gasket and punch out water holes. Use a bit of RTV around the water port holes
Bprojects $13

Nissan common A14 gasket



GX gaskets