The primary reason to own a GPX property (imo) would be to use it. The locations in Southern CA are awesome. We bought within GPX primarily because of location (Seapointe, Coronado) but have found the day use to be our favorite feature. We're down at Coronado almost weekly. The trading power in RCI has been all over the place in the past few years (my Seapointe week 32 1 bed has gone from 50 to mid 30's - then mid 40's) but there are many more cost effective RCI traders out there. I wouldn't buy this as a trader. As far as the maintenance fees, I think that they are on the average to high side, but the quality has definitely increased recently with the new renovations.
I have heard that there is GPX preference on RCI but have not tested this. I am thinking of giving GPX a try with our Coronado week and try to exchange into Hanalei Bay Resort.
Btw, I have definitely seen summer weeks on GPX in the past, but I think that it's hit or miss. If I recall correctly, I think that I saw summer 2017 weeks earlier this year - maybe late winter or early spring. Presley mentioned that she didn't think that the ongoing searches in GPX were automated, so you are rolling the dice when requesting. The bonus weeks are awesome. I sent my dad and his wife to the Ka'anapali Beach Club about two years ago for about $500 all in. They were even given a partial ocean view room.