A summary of my thoughts/observations on Gamma World after working with it for most of the evening:
I need more time with the GW monsters to really get a full idea of how they scale but I can't see any reason why you couldn't do cross overs in either direction. The monsters are formatted almost identically but the power levels feel off. Throwing D&D monsters into GW or GW monster into D&D seems entirely possible.
Ammunition works as previewed: If you use an ammunition-based weapon more than once in an encounter you are out of ammunition after that encounter. It can only be replenished by a random junk roll or by the DM (via loot or trading or what have you.)
There's no currency, and mundane equipment is simply taken as you wish at character creation. Armor is basic (Light and Heavy Armors, Shield) Weapons are categorized by weight (Light and Heavy), by style (One or Two Handed), by range (Melee or Ranged) and by whether they're firearms (Using Ammunition.) It's left up to the player to decide how to describe what they have: A one handed light melee weapon could be a machete or a pair of scissors. A one handed light firearm could be a Glock or a Derringer.
There are junk items (including things like decks of cards and Twinkies) that appear to be trade or roleplay fodder as none have outlined functions or rules. Even some gear in the character creation section is simply tagged 'You'll figure out a use for it.'
Each origin has a defined set of Powers, every so many levels you choose a new Power from your primary or secondary origin. Alpha Mutations act like additional Powers and are changed frequently (after an encounter, a natural roll of 1, at the start of a new session, etc.) Alpha Mutations that compliment your origins are easier to overcharge and use.
Omega Tech are like Encounter Powers and Magic Items combined and are 'tapped' (yes, like in Magic: The Gathering) when used. At the end of the encounter you roll to see if the item is depleted, if not it's untapped and can be re-used. If it is depleted it can be salvaged (which turns it into a permanent but less powerful item) otherwise it is lost.
Omega Tech is drawn at the end of 'most' encounters (DM fiat.) When a player has his or her own deck of cards, there's a roll to determine whether they draw from their deck or the DM's deck. The DM is supposed to use all of the cards in the boxed set as their base decks but may add cards.
My observation is that the game is designed from the ground up to be open ended and humorous. There are jokes mixed in with rules explanations and the DM guide amounts to a basic explanation of how to design an encounter and several reminders that the game isn't a competition and that the objective is collective storytelling. There's not much in the way of rules for non-combat situations. No such thing as a Skill Challenge for example.
There's an emphasis that Gamma World campaigns will tend to be short and messy. Monsters appear to top out at level 6 or 7. Alpha Mutations and Omega Tech are definitely on the goofy side and would have potentially serious balance issues in a D&D game. From my experiences so far I'd call Gamma World the 'Grindhouse' version of 4e, but I intend to run at least one mini-campaign before finalizing my feelings on the game.
