For your purposes
Welcome Ross.
This is not really practical using gps hardware. Read about Accuracy and the smallest distance a gps receiver can resolve in Using the Location Sensor
If you want to 'try', here is an example that would work if the distances are greater than a few meters and uses a real time database Social Distancing ... a CloudDB / Location Marker Tutorial and tells you how close users are to one another.
So what to do:
1.) use a proximity sensor if all player's Android's have the hardware ProximitySensor There are video examples.
I'm not sure whether it is possible to determine which device it is near so it might not really be practical to use a proximity sensor either.
2). you might be able to use NearField sensor . NFC Cup Game for AI2 You would have to provide each player and purchase readable, writable, NFC tags. It will tell you if the NFC tag is range of your device (if I remember, the tags need to be within a very short distance of the Android).