Hmmmm I had no idea but a quick Google
G4 is dismantleable for cleaning- and seems to be a pretty chunky robust blade with a secondary bevel (is that the right word?)- of 30 degrees.
G5 is non dismantleable- that has a consistent 20 degree angle across the blade it entirely ?
G6 is a dismantleable version of G5
Again I won't pretend to properly understand - but from what very little I know- the g5/g6 blade is more shaped and angled like a knife for finer work, food prep, meat cutting, skinning etc- but probably more prone to becoming blunt if you hit much bone.
The G4 30 degree secondary angle is more suited to robust tasks- woodwork, mild bushcraft etc etc
I had a G4 and found the blade too chunky for deer work. Not because of that secondary bevel thing- but because of the shape of the blade.