Below is a screenshot taken from a 1942 Chevrolet Trucks document that describes the bed constructions as "Heavy-gauge steel". Probably the same as your 1946 GMC as they shared many of the same features and construction grades.

Mar-K who is the go-to for reproduction beds for our Chevy and GMC trucks lists their bedside panels are made from "16-gage electro-galvanized steel".

Mar-K only lists 1 size front drivers/passenger side and drivers & passenger side rear stake pockets for all their bed side panels and those are also listed as constructed from "16-gage electro-galvanized steel".

Perhaps one of our fellow GMC Bolters can chime in with more detailed info to answer your question???
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)