Gamma World Online (GWO) - no, it doesn't exist yet, and Wizards of the Coast has no plans that I know of to develop it. But - why don't they? Given the enormous popularity in post-apocalyptic games/movies/series, you'd think that GWO would be developed by now. We saw what the Fallout series did, but that was one-player, and Gamma World as a concept pre-dates Fallout (i.e. no argument from Bethesda on infringement).
What say you? Do you want to see this WOC owned game line become an online MMORPG?
My opinion - Gamma World as a concept would actually work better as a video game/MMORPG than the original paper/box game it was developed as.
In case you have never heard of Gamma World - do a search online and view the history, but - it was a TSR published game under Gary Gygax's tenure in the company, but Gygax only wrote one portion of a product and was not the primary author of this line. James Ward is the primary author and developer of the game, with a predecessor game called Metamorphosis Alpha being his first game that heavily influenced Gamma World (WOC does not own Met. Alpha though, as I understand).