I only parsed that link quickly but here are my thoughts;

It appears that the GPS units are Motorola Oncore units. The linked site doesn't explicitly state which model most of them are but infers that they are the Oncore GT, and the visual evidence would concur with that. It does note that newer models may be Oncore M12 units.
The GT units work fine with gpsapp. I have one in my car. However a couple of things should be noted.
Firstly, it initially boots into the Motorola Binary mode, not NMEA. gpsapp does not (to the best of my knowledge) currently provide a way to switch modes, meaning that the mode switch must be done with a PC app, and also that battery backup power must be maintained after doing so.
Secondly, it appears from that site that the owner is hacking the serial cable straight into the 5V TTL level signals on the GPS board. While most PCs (and empegs ) can reliably understand TTL level serial communications, this system affords absolutely no protection to the Oncore unit, and I don't believe that interfacing a 12V serial port is going to be good for it. Perhaps there is some level of in-built protection, but it is not documented, and should definitely not be assumed.

A better solution to use that board might be to search this BBS for 'Oncore'. A while back I posted a printed circuit board layout that the Oncore GT can mount to that will break out the necessary signals and buffer them. This would also leave your main Onstar pcb stock, thus eliminating the chance of damage that could hit the vehicles resale value.