Really confused about compiling my own GPI0 SSDT.

I've been following the OpenCore guide, but this part has went over my head. I compiled and decompiled the DSDT or whatever, I've found the part about the GPHD == zero, I've downloaded the SSDT-GPI0.dsl, and I'm just lost as to what I'm supposed to actually edit so I can recompile.

What obvious information am I missing here?