A gadget is a small tool such as a machine that has a particular function, but is often thought of as a novelty. Gadgets are sometimes referred to as gizmos. Gizmos in particular are a bit different than gadgets. Gadgets in particular are small tools powered by electronic principles (a circuit board).
A tool is any physical item that can be used to achieve a goal, especially if the item is not consumed in the process.
Tools that are used in particular fields or activities may have different designations such as "instrument", "utensil", "implement", "machine", "device," or "apparatus". The set of tools needed to achieve a goal is "equipment". The knowledge of constructing, obtaining and using tools is technology.