I'll measure carefully tonight if it isn't stamped or stickered right on the box itself.
I tried to get a picture of a piece of the steel up against a ruler, but could not get a good shot.
At any rate, I measured the thickness and it is a tad over 2.5mm. According to this table (http://www.mesteel.com/info/carbon/thickness.htm), that puts my box at 12 gauge. It's not 1/8", but it's pretty goddamn thick.