There are all sorts of reasons for wanting to have a GPS unit attached to things so you can find out where they are or where they have been. As far as I can see, if something belongs to you tracking it with GPS is legal. Tracking other peoples things not so much.
There are all sorts of hardware setups, there are small tags that can be clipped to a pets collar or kids backpack. Devices like the picture above that can be permanently or temporarily attached to a car. The hardware can be a smartphone.
All require some software and for real time tracking, there may need to be a cell phone simm card and a subscription service (maybe $25 a month). For after the fact tracking, there are some that simply store the GPS data on a mini SD card you read later on a PC. There are even some that use bluetooth and can send a real time update of location through other peoples (participating) bluetooth phones.
The vehicle attached units can have other useful options, especially for watching where your teenager is taking your car. They can send SMS text messages to your phone if they arrive at or leave certain areas home or school or work. They can let you watch real time and see the location and speed of the car.
The ones connected to the cars systems can tell you other things like low on gas or oil, if the unit gets removed or disconnected. Some can even disable the vehicle and prevent it starting. Some can pick up and transmit to your phone sounds in the vehicle (recording conversations may violate state laws).
There are apps that can be installed on a smartphone that will track the phone some secretly some openly these can be useful for tracking business travel or tracking teens. They can also find the phone if it is lost, some can activate the phones camera to capture the person who may have found or stolen the phone and send that picture to you. They can also disable the phone remotely.
There may be some legal questions about tracking an adult or their car or phone. But tracking your own pet, car and phone should be OKAY.