Gimkit Creative has a small list of approved icons you can use. Anywhere you can enter a Custom Image URL, you can enter icons/NAME OF ICON and that icon will be used.
For example, to use the Add Energy icon, you would enter icons/Add Energy into the device.
Add Energy
Add Energy
Efficiency
Efficiency
Endurance
Endurance
Speed
Speed
Piggy Bank
Piggy Bank
Add Cash
Add Cash
Multiply Cash
Multiply Cash
Cash Stack
Cash Stack
Backpack
Backpack
Bolt
Bolt
Stopwatch
Stopwatch