I've always washed GTX on a very cool wash using pure soap in liquid form, then big-time rinsing - I usually put 'em through the machine a second time without soap. After that's all done put 'em in a dryer on medium hot for about 40 minutes and they're good to go. The heat from the drier has an effect on the fabric itself and any DWR that might be on it and re-juvenates it all at a molecular level, they tell me...........
I can't see how a 'proofing can do anything but harm to GTX, it's the physical properties of the material itself that does the job not some kind of coating or treatment; anything you put on or in it can only stop it from breathing, surely? Otherwise you'd just treat some much cheaper cotton or poly-cotton and get the same result, as I see it...........................atb mac