I plan on running a brief Gamma World campaign with my group, and I am wondering if 4E monsters are compatible between systems without any sort of level adjustment? From a quick perusal, it seems like PCs are considerably more powerful than 4e characters of equivalent level: adding level rather than 1/2-level to rolls, alpha mutations/tech cards, major critical bonuses, hit points are restored with a short rest, etc. At the same time they are much less "survivable": the only reliable healing is through second wind, no specialized leader roles. To be equally challenging should I treat the party as a few levels higher when building an encounter with 4e monsters? Any advice would be appreciated!
D&D 4E and Gamma World encounters can be used almost as is in the other game
Characters in Gamma World dish out more damage, and are a bit more brittle than their 4e equivalents, but that just makes fights a bit more swingy and a run a bit faster.
D&D 4E and Gamma World monsters can be used almost as is in the other game
Comparing some monsters in the two games
D&D 4E, Monster Vault
Also see this Extreme Reskinning article for an example of reskinning a white dragon from the Monster Vault into a cyborg assassin for Gamma World.
GW plays loose where 4E is stricter about balance, so just wing it and see what happens. Maybe it's a cakewalk (add reinforcement baddies) or maybe it's a bloodbath (various options), but as long as everyone has fun it's all good.