since the newest firmwares can send a tracker device to sleep and safe a lot of energy, i guess also esp32 devices qualify for the role of low-energy-gps-tracker devices.
before tracker-sleep, this was far-best with Rak-devices…
but since maybe cheaper and smaller ESP could do the job… what devicis are available? i only have experiences with tdeck and t-beam, which both are not ideal to be small gps,tracker 
t-beam might do the job, but there has to be something cheaper, smaller without display :), maybe to be glued into epoxy resin 