The Nano Gear keychain is obtained after you beat San Fransokyo, the Big Hero 6 world. It resembles Hiro's willingness to let go of his past mistakes by destroying the original chip he made for the first Baymax.
The design has nanobots coming out the top of the keyblade with a bit of Baymax's armor holding them. The head of Hiro's bot fighting robot holds the blade and grip for the keyblade. The ring around the grip looks like Baymax's wings and feet thrusters with a sort of target at the bottom. The chain looks like lanterns connected by string with the healthcare symbol on the healthcare chip Baymax has.