Gadget can withstand extremely high temperatures. Plutonium has an exceedingly high boiling point of 3228°C. This is a physical property.
If she holds an object for long enough, it will begin to burn in her hand. Plutonium metal is warm to the touch, because of the energy being released from alpha decay. This is a chemical property.
Gadget's abilities break when being overloaded, and they're attempted to be used on multiple things at once(ex: surrounded by a room of other highly reactive materials). Plutonium is highly reactive, and can be affected by almost anything. This is a chemical property.