Because a battery is capable of a large current capacity doesn't mean that you have to use it. It's like a car... goes fast If you tell it to, not because you have to.
If your GPRS sheild requires 2 A that means that it draws 2 A or 10 watts of power which is .83 A at 12 volts so you might be able to operate for 7 hours with a switching regulator. The calculation is like this calculate the power in watts at the required voltage (2A X 5V = 10 watts... so we need 10 watts from the battery and a switcher is "About" so from the battery we need 10 watts + the efficiency (.8 - 1.0 X the required power is .2 watts) + 10 watts = 10.2 watts or .85A from the battery to supply 5V @ 2A. The .2 watts is lost as heat. With a linear regulator you would have a 2A load at 12 V or 24 watts from the battery and 24 - 10.2 = `13.8 Watts lost power in the linear regulator as heat. so with a linear you loose 57.5% of your power as heat... Not so verry good. The buck mode or step down switching regulators can be purchased for about $1.75 from Ebay and I would use 2 one for the 2 A load and one for the 1.2 A load (approx) for the Arduino. Amps is like gas tank capacity20 - 40 - 80 liters... but you only use what you need. Volts is like Octane if it's too high you go too fast and then you might crash and burn...
Doc