Gadgets are type of weapon that can only be used once. They are unlocked at level 8. They can be bought in the shop for cash or gold in packages of 5, 10, 20, or 25, depending on the weapon. You can only use one type of gadget in a battle, but it can be used repeatedly as long as you don't run out of them. However, there is a cooldown, which varies on the gadget. To use a gadget, you press the button in the top right under your weapon.
These are all of the different gadgets: