GN, GP, GL etc. is just telling you from which generation the monitor is.
GL = 2019 models
GN = 2020 models
GP = 2021 models
The 34GP83A is basically the newer model of the 34GN850. Idk why they changed the 850 in the name to 83A in the new generation. Most differences are subtle because it's basically the same hardware with a few software tweaks (mostly just the new OSD).