The McWill GBC Mod includes the possibility to mod your GBC to run off a rechargeable LiIon battery which can be charged via USB (an it's recommended to do so).
This battery is fully compatible with this modifictaion. One battery is sufficient for operation.
(ATTENTION: Even though the battery is AA size, it is a 3.7V LiIon battery! So do not try to use it as a replacement for normal AA batteries or even charge it in a normal charger!)