Mass for a fixed area
GSM means grams per square meter, written g/m². A 1 m × 1 m piece of 100 gsm material has a mass of 100 g. A 0.5 m × 1 m piece of the same material has a mass of 50 g.
Paper and fabric
Unlike US paper basis weight, GSM uses the same area for every paper grade. It is also used for fabric. A greater number means more mass for the same area, not necessarily greater thickness or stiffness.
For printables, use paper supported by your printer. Check its media specifications rather than assuming that every paper of a given GSM feeds the same way.