Yes you will, the battery shape you currently have is known as a "Nunchuck" battery. You will need your battery to be in a "brick" shape.
There's 2 ways you can do this, if you're confident in your soldering abilities/electronics knowledge, take apart your nunchuck pack and rewire it into a "brick" that fits in your PEQ box.
OR the second way is simply just buy a new battery and either keep your current nunchuck as a backup or sell it to someone or something.
seems like a pretty good site for buying a new batterypack (Apparently they're owned by or a subsidiary of CheapBatteryPacks.com
which a lot of people know and recommend so there should be no worries about ordering from them).
EDIT: One thing I should mention is that with the Elite Airsoft Batteries site the "PEQ Packs" are 9.6V, with a higher voltage you get better trigger response and all that but obviously running higher voltages isn't that great unless your gun can take it (I have no experience with the G&G and 9.6V so best to wait for someone else to chime in). If you don't feel comfortable about that then go to Cheap Battery Packs and choose the "custom NiMh/NiCd packs" option and make an 8.4V pack that will fit in your PEQ box.