There are 2 types of currency in Dead Trigger 2: Money and Gold.
Money: Money is the basic type which you get as a reward for playing missions and completing goals during each mission. Money is can be spent on buying or upgrades for your weapons, gadgets or upgrading your crew.
Gold: Gold is a premium currency that is used speed up upgrades. You can buy gold in the ingame store or get it for free through “Free Gold” button.