Our 10, 15 and 20gal kettles are made from a 1.2mm (18 gauge) 304 stainless steel (18/8) and our 30 and 50gal kettles are made from 1.5mm thick 304 stainless. Based on what's out there we have the thickest kettles on the market. Many entry level kettles are .8mm (22 gauge). For reference a stainless steel beer keg is about 1.5mm (16 gauge) and is meant to be shipped around the country and beat up by drunk college kids.
As for the 6mm thickness... When you see 2, 4 or 6mm thicknesses for a kettle it is made from aluminum. Aluminum is much lighter and less tough than stainless steel. A 6mm thick aluminum kettle will dent much easier than a stainless kettle only .8mm thick. When it comes to kettles you really get what you pay for.