Civic EG

When installing a Z6 head onto the 15b7, which head gasket should I use?

Either a z6 or a d16y8. The y8 will yeild a slightly higher compression ratio

or more thiner head gasket 0.030", cometic or something like that, to raise compression ratio more, it's thiner then stock d16y8/z6

Swapping to the z6 head with a z6 gasket already nets you 10+:1 CR. Just use the z6 head gasket if you want to bump the CR a bit, have the head milled while it's off the block.