1 GSM = 0.0426 mil if the density is of Polyethylene or 0.92g density.
1 mil = 23.474 GSM
m2 (1) g 30,000 in2 mil lb (30,000)
_____ x _____ x ________ x ___ x ____ = MIL _________ = 0.0426
1550 in2 m2 lb 1 454g (1550 x 454)
Example 60 GSM = (60*30,000) / (1550*454) =2.56 mil
60 * .0426 = 2.56 mil