According to Wikipedia, standard wire gauge is a little different from sheet metal gauge:
http://en.wikipedia.org/wiki/Sheet_metal#Gauge
So 16 gauge steel sheet is actually 1.52 mm nominal, and 1.39..1.64 is the tolerance range. The next thinner gauge (17) is 1.422 mm for wire and 1.37 for sheet, so I would say that 1.5 mm is the metric equivalent of 16 gauge.
I made my chassis out of Czech made 25x25x1.5 box, black, S235 grade, but its actual thickness is 1.6-1.75 mm.