There is no such thing as an 8.4V Lipo battery. Lipos come in 3.7V stages, so if you have a normal Lipo it would be a 7.4V (two cells). The next up is an 11.1V Lipo. Despite being a good gun a G&G might not take too kindly to being run on an 11.1V Lipo, as it would up your rate of fire to probably more than the gearbox could handle.
If you definitely have an 8.4V battery then it's either Nihm or Nicad, not Lipo.
If you want a bit more power then, depending on whether you keep the battery in the stock or the foregrip, you might be able to use a 9.6V NihM without causing any undue problems to the gearbox, either crane stock or sub-C cell size of about 2200mAh capacity, unless you have a solid stock version, in which case you could probably get a 3700mAh in there easily, which will last all day at most games.