From publisher blurb:
This is the main PDF, clocks in at about 30 pages, and includes everything you need to create a Gamma Five player character:
Bios: The "bios" are the equivalent of a race choice, but they work in a different way, determining some of your ability scores and how you roll the others (usually 3d6 roll), and providing you with an array of "set" mutations, plus a number of randomly generated ones.
Mutations: The document includes 50 different mutations, all of them beneficial. I know that, to some, this might be bad: personally, I hate randomly generated defects (the 3d6 ability score rolls are quite punishing by themselves), and I felt like 50 mutations would be a good number. Also, this system is a bit more flexible than 7th edition's origins (which I like, by the way).
Gamma Wanderer class: In Gamma Five, PCs all share the same class. The Gamma Wanderer class goes up to level 10 only, but climbs up quicker than D&D Basic classes when it comes to features (e.g. 2 extra attack features by level 9). It also grants proficiency in all weapons and armors, plus a d12 Hit Die and four skill proficiencies of the player's choice. While clearly unbalanced in a D&D Basic game, those benefits become a must when facing Gamma Five monsters, which are a bit tougher than their Challenge Rating might suggest.
Skills: Gamma Five leaves out some skills (Arcana, Religion...) and introduces new ones (Ancient Tech, Conspiracy, Science).
Equipment: The obligatory gear section. In Gamma Five, weapons and armor are handled in an abstract way. You simply choose a type of weapon (Light Melee, Heavy Melee, Light Ranged, Heavy Ranged, Light Ranged Gun, Heavy Ranged Gun) and a type of armor (Light, Medium, Heavy, Shield), write down its fixed stats, then you decide what it looks like. This comes straight from the 7th edition of Gamma World, a design choice I really loved.
The Equipment chapter also includes random charts for scavenged junk (100 items) and Ancient Tech (20 items).
Running the Game: This chapter is very short, and contains some troubleshooting advice and some examples of interesting terrain for a session (laser grids, debris, plasma pools, nanobot swarms...)