1) Should I use GPS after Hyper Polish to enhance the gloss before using Gloss Coat?
No dont use GPS if you want to apply Gloss coat. GPS has some built in protection that will interfere with Gloss coat bonding to the paint. Hyper polish alone gives some mad gloss on its on. Also I would suggest you buy and use some OPT paint prep before you apply the coating.
2) Can you use their spray car wax occasionally to enhance gloss & UV protection over Gloss Coat?
Yes ofcourse. OCW, opti-seal or hyper seal are all good ontop of Gloss coat.
3) What can you use to prolong the Gloss Coat's protection? Just Opti Seal or other products like the detail spray etc?
If you get the newest version of opti-seal its got some SiO2 protection built in so thats good for topping up...the old version is fine as well. Opti coat hyper seal is also another option for a topper.
4) Is it OK to use their car wash when necessary or will it hurt the gloss coat?
Yes its perfectly fine. OPT synergy remember!
The car will get contaminated with iron particles over time so I also advise you use Ferrex & clay towel every 6 months. So basically wash, spray ferrex liberarly, use a clay towel to spread ferrex around the car, rinse the car after a few minutes and use some sort of protection product to dry and protect your paint.