On a lot of European 4x4s and workhorses, G stands for Gelände, German for terrain. It is a super short crawler gear that sits below first, meant for mud, steep climbs, towing, or inching over rough ground at walking pace.
Think of it as granny gear. You use it when you need control and torque, not speed. On tarmac in normal driving you start in 1 and ignore G unless you really want the gearbox to hate you.