I am going to throw you for another loop. If you are stuck on the GPX, you don't necessarily
need to upline. The half line weight should be fine to chuck moderately weighted payloads. If you want the UTT, consider uplining.
I really never asked what the payloads you would be throwing with the rods.
For my true nymph rig, I use a Steelhead taper which have the standard line mass in the first 30 feet. On my 6 weight Z-Axis, I use a 7 weight SA Mastery Steelhead line. The extra mass turns over the heavy
rigs I throw, plus the long belly helps me make big mends. I will throw a "spark plug
" nymph with my 6 weight.
I use a WF5F Sharkskin GPX for my 5 weight Z-Axis as a general purpose line. It is a decent nymph line. It mends well. I have to keep my drifts within 40 feet because I lose line control on anything longer. I also limit my payload. For example, I won't be throwing a "spark plug" nymph with my 5 weight. The flies that I throw on this rig are #8 and smaller with modest amounts of lead.
I also didn't ask the manner that you fish.
I fish out of a drift boat regularly. It is important to have a line that fires off quickly. If I waded, I could get a little more load with a water load.
Sorry if there was any confusion.