There’s a guy here who is already playing around with a long distance drone project with GSM and a server on the internet as a fail-safe and way to send commands when the RC connection is lost, maybe that helps:
http://community.emlid.com/t/my-next-build
I can run my Pi2+Navio all day long as a computer with a 2000mAh 3 cell battery. So if you just need a few hours of tracking I imagine a much smaller 2 cell battery with low C rating would do the job. To calculate that you’ll need help from somebody else and use some kind of battery calculator tool.
I just measured my development RasPi2-Navio+ for you, it’s drawing 0.22-0.43 amps (depending how busy it is), 4.9-6.55 watts, with an ESC as BEC, servo, RasPi camera and HDMI cable connected to an active monitor. That’s just a quick snapshot, actually out of interest I’ll try to do some extended tests another time as I’m interested in the same thing. I’m writing some software right now which interacts with each chip, and noted some of them have a “sleep” mode. Such a tracker/failsafe program should take advantage of these modes to maximize backup battery life. Maybe I can help there later, it’s an interesting idea for somebody to develop.
Good luck. If you need another power measurement with a specific configuration of standard hardware (I don’t have a GSM module) and software running I could do that for you.