Strictly speaking Gun Bros uses the freemium model - as you play through the game you can pick up Xplodium, which can be refined between games for weapons upgrades, armour, and special bonuses. This takes a lot of playing time, so you can bypass the process and simply buy the resources you need with cold hard real world cash.
Rest assured you can finish the game without investing money, but it won't be as easy to beat the B-movie plot. Oh yes the plot, it's bonkers... from the directory listing:
Like a super-powered fungus gone haywire, the evil T.O.O.L organization has grown from the dankest levels of the criminal underworld to spread its own brand of malevolent funk to every corner of the galaxy. Most recently, they’ve set their sights on harvesting the rare element Xplodium in an effort to create even more powerful weapons to enslave the universe… But not if the Gun Bros have anything to do about it!
Looking for the download? It's in our App Directory, or in the Windows Marketplace on your handset.