A green power ring is basically a “I’m God” button so it’s not really a fair fight for a gundam. Generally speaking the only thing that stops ring-wearers is other ring-wearers which is why there is a spectrum now.
The most powerful Gundam in the Gundam multiverse is the Turn A Gundam. It is powered by a black hole. Controls enough nanomachines to coat the solar system. It can turn just about all matter into sand and regenerate totally regenerate itself and pilot from it’s nanomachines. It takes time but is basically immortal nonetheless. Also possibly continually evolving as it is speculated to be a highly evil Devil Gundam from G Gundam.
It could probably kill most of your average Green Lantern including John Stewart. Hal Jordan or a Ion or Parallax it gets questionable. The ring would really need to be taken to it’s absolute limit. The rest of the League doesn’t stand any chance. It can just turn supes into sand.
It was specifically designed to be the end of the tech tree for all current and future Gundam series. As Gundam is a “real” robot series that operates on Star Trek level real science. Maybe even more realistic as travel outside our own solar system is never achieved. It also basically an imagining of what is the absolute theoretical limit of human technology.