NMEA is 4800bps, so that would be less than 5% of your original estimate based on 115kbps, so maybe 2GB.

That said, nothing is mounted r/w on the empeg while it's in the car. You could make it mount something r/w, but you risk corrupting data when you shut it down.

Also, the empeg will spit out data on the serial port on boot. It probably won't brick your GPS, but I don't know that I'd trust that. I think Mark has added some stuff to Hijack to allow app support for the serial port, which is usually the console, but I don't know how that all works.

Honestly, I'd be inclined to either use a laptop, if laziness is the top concern, or get some single-board PC with a big-ish flash disk running Linux if it's not.