5V*2A*10h = 5V*20Ah = 100Wh. You could use a 6V 20Ah NiMH battery pack and a linear low-dropout regulator.
Using a 12V battery and a switchmode regulator with 90% efficiency would require a 100Wh/0.9/12 = 9.26Ah capacity.
But I guess that it has an internal battery and that the 2A spec' includes the maximum recharge current for this, thus the gps may not draw 2A on a continous basis.