Seems like a challenging project and if your looking to buy something like pocket finder, I would just buy their product. however if your planning it as a DIY than you need to look for a small, low power gps, a micro to parse the data, battery, and a transmiter (probably cellular). this isn’t too hard to put together but getting all that to the size of an oreo is going to be a little tricker. I would probably create my own PCB to get everything as compact as possiable and use smd components.
since your new to gpss heres a little info on gpss:
A GPS works by detecting the signal of a minimum of 4 satalites which are transmitting the time and their location. The reciever calculates its position through finding how far the satelites are and then calculating its position.