Gquuuuuux is it’s own timeline so you don’t have to watch anything to get up to speed, but if you’re watching Universal Century, watching it in release order would be the way to go. Skip Igloo, Requiem, and ZZ if you want. I finally watched ZZ on my third UC rewatch two years ago, and it made no difference in how I viewed the later series. ZZ is a huge waste of time.