There is no universal fundamental to convert GSM into MM, however u can try by using this formulae
Thickness of the paper (mm)/0.13= GSM

FYI,

International Metric Paper Sizes - ISO Standard
SIZE
------------------------------------------------- -------------------------------
WIDTH (mm)
----------------------------------------------------------- ---------------------
HEIGHT (mm)
----------------------------------------------------------- ---------------------
WIDTH (in.)
---------------------------------------------------------- ----------------------
HEIGHT(in.)
---------------------------------------------------- ----------------------------

A0 841 mm 1,189 mm 33.11 in. 46.81 in.
A1 594 mm 841 mm 23.39 in. 33.11 in.
A2 420 mm 594 mm 16.54 in. 23.39 in.
A3 297 mm 420 mm 11.69 in. 16.54 in.
A4 210 mm 297 mm 8.27 in. 11.69 in.
A5 148 mm 210 mm 5.83 in. 8.27 in.
A6 105 mm 148 mm 4.13 in. 5.83 in.
A7 74 mm 105 mm 2.91 in. 4.13 in.
B0 1,028 mm 1,456 mm 4 ...more