Yes I agree with what has been said 4mm gauge wire is good for the DCC track bus.
Many of the DCC books including the one by Stan Ames (and some websites) have tables in them to show what gauge wire to use to keep voltage drop to an acceptable minimum for various lengths of bus and central station power. These are mostly in AWG, which you then need to convert.
I use standard 4mm, solid core, house wire singles, purchases in large 100m length drums from any good electrical distributor.
For runs over about 10m I've have gone to 6mm gauge house wire.
Get two different colour wires and twist them together, stick to the same colour scheme throughout. The twisting together not only keeps the wires together but will reduce radio interference. Obviously the different colours help to identify which side of the track a wire should be connected to. Some sort of UV protection is a good idea. All of mine are buried for this reason.
Since G Scale track has a much bigger cross section than the wires used to get power to them you do not need extra feeders for voltage drop reasons, especially when using rail clamps throughout. However extra feeders may be of benefit where conductivity at joints could be suspect, where sectionalisation is needed, for example where there are multiple power districts or a need for detected sections for computer control, or where you need to connect up some accessory such as a reverse loop unit.