Bolts: This is the basic currency/money in Gears&Guts. It can be earned by playing missions, daily rewards or trading tokens (iOS) or Glu Credits (Android) in. There are several weapons and cars in the Store which can be purchased if the player has enough Bolts. Bolts are also used when upgrading weapons or cars.
Tip: If you often find yourself low on bolts, the easiest (and cheapest) way to get them is to play a non-timed mission such as 1-4 and just keep killing zombies for as long as you like.
Glu Credits: If you are playing Gears and Guts on android then this is the currency you purchase for real money via Google play. You can also earn it randomly from drops upon finishing missions or completing various offers. Since Glu Credits are used and shared in several of Glu Mobiles android games, its possible to earn them in other games and then use them in Gears and Guts if you so wish.
Tokens: If you are playing Gears and guts on an iOS device, this is the currency you purchase for real money via the ingame store. You can also earn it by other means, such as reaching certain milestones within the game or completing various offers.
Both Tokens and Glu Credits can be used to buy certain weapons and vehicles as well as permanent boosts (double weapon damage/double bolt gain), revives and Bolts.