I have all of the necessary wiring for hookup. I am placing the gfci disconnect on outside of the house but I have to run over the existing patio to tub which is approx. 8-12 ft. Can I run #6 thhn wiring in grey plastic tubing and have it exposed (I don't know any other way without busting up the concrete?

Your plan:

Can I run #6 thhn wiring in grey plastic tubing and have it exposed

Will be fine.

Be sure the THHN is also rated THWN, for wet locations, which is outdoors.

Be sure to use at least a #10 THHN/THWN green insulated conductor for the equipment ground, along with the three #6 conductors.