What is a gun belt?
A gun belt is meant to hold a pistol, and enough ammunition to support that pistol. Also, you can mount some bare essentials to it as well, however these should be seen as extra.
At the very least, you will need a holster, and a few ammo pouches. In this video, Lucas also included a multi-tool in a pouch, a tourniquet, a knife, and a dump-pouch.
What is the difference between a gun belt and a war belt?
A war belt, unlike a gun belt, is meant to hold mission essentials as well. This can include IFAKs, small GP pouches, grenade pouches, and other items.