We are replacing our hot tub and need to install 50A 220v GFCI. ( the old tub had internel GFCI ). We have a sub panel @ 40' from tub on back deck with the current 50A dbl pole breaker, It is supplied by a 1 - 6 gauge black and 1 - 6 gauge white and a bare ground to the ground ( ? ) bar in the panel.. to supply the tub I have 1 6ga red.. 1 6 ga blk.. 1 6 ga white and 1 6 ga geen,, I'm confused on the where to run the white pigtail ( curly wire ) from the GFCI in the box... thanks for the help!

You 100% absolutely CANNOT install the GFI breaker in the disconnect panel. It MUST be installed in your main panel because there is no neutral in the sub and the GFI breaker requires a neutral to function.

Does your tub require the neutral? Is it 120/240v?
Meaning do you need two hots, neutral and ground to the tub?

If so you CANNOT use that circuit to the sub-panel for the same reason as above.