While it is perfectly legal to put a GPS tracker on any vehicle you officially own (either as an individual or through your business), you might not be able to put it on someone else's vehicle. It varies by state. In most states doing so without permission is considered a violation of the vehicle owner’s right to privacy, and you could face fines or even criminal charges. Not all states, so it's best to check with a lawyer and/or the laws in your state.
overall, If you want or need to track someone else’s vehicle, make sure you get that permission recorded or in writing. This helps to avoid any future issues, complaints, or complications.