The actual access to the GPS hardware is probably done by /usr/libexec/location-daemon. Unfortunately this file does not open any files in /dev.

My guess is that communication with GPS hardware is done over phonet0 network. I do see a lot more action there (use tcpdump -i phonet0) when GPS is in use, but it possible that this data is used merely for GSM cell based location.

As for the data format - it seems that location-daemon is using liblas.