Gearing should always be lowered via the differential if you're taking a CUP setup to GNS.

Then see how the car runs (remember 1 change at a time).

There were some other minor adjustments that I can't remember off the top of my head now, think it was like tape or shocks or something.

But the gearing is the big thing. VH has some great CUP setups that would make good base GNS setups.