Need to know how to hook the pair of 6 3 wire together with gfi breaker. Its for a hot tub I have both sets of 6 3 wire ran just need to hook together with gfi.

The GFI breaker/subpanel has Line lugs at the top and load lugs on the breaker.

The cable from the panel is Line, the cable to the spa is Load. Black and red connect to Line and Load.The Line white neutral connects bar the neutral bar, the Load white connects to a silver lug on the breaker, this lug is usually next to the white pigtail wire on the breaker, the pigtail connects to the neutral bar.

If this is cable has a bare or green groubd wire, they both connect together on a lug or bar that is separate from the neutral, and bolted directly to the panel metal box through a threaded hole(s).

Connect it up and take a couple of pictures to post here so we can check it.

This needs to be absolutely correct since your heinie will be in the spa.