Since the generation of electricity seems to be the biggest problem here, I decided to search for materials that could make this happen.
The most interesting thing I found was published in Nature just a month ago. Scientists developed an elastic waterproof material that can create electricity based on the magnetoelastic effect. The mentioned effect is described as "the variation of the magnetic properties of a material under mechanical stress". Basically, by even the slightest movement of the elastic band such as the pulse, small magnets in the silicone matrix change their mutual distance and therefore, result in a change in the magnetic field. These changes are turned into electricity.
I guess people will soon start implementing these into wearable gadgets such as fitness bands, smartwatches, and smart rings. I don't see why it couldn't work in the case of GPS trackers, too. :)