The big advantage to using Nikwax/Grangers/HG is that they don't contain detergents as normal wash powder does. Detergents clog the pores on GoreTex, reducing the breathability you paid so much for in the first place. If you can find 100% soap flakes these work just as well.
The best way to treat them IME is to run the washing machine empty on a hot cycle first to clear all the detergent residues. Then a cool wash with your preferred fabric wash (Grangers in my case) followed by another cycle in the machine with a re proofing agent (Grangers again). Finally a gentle tumble drying which is recommended by WL Gore to rejuvenate the membrane. Take all the armour and liners out first if possible. The thermal liner gets washed as normal clothing whenever it gets a bit whiffy.
It's all a bit of a faff but I only do it once a year at the end of the winter, and all my walking, climbing and Mountain Rescue gear gets done at the same time.