GSM stands for Grams per Square Meter, a unit for measuring fabric weight.
The GSM can also be calculated by using some machines and apparatus. But, the simplest method of calculating GSM is to cut 10*10 cm of any fabric and weigh it. It is then multiplied with 100.
For eg. If the weight of the fabric (10*10 cms) is 2 gms, the GSM of the fabric will be 200.
The other numerical formula for GSM is stated below:

Weight per Square meter     =
(in gram)
Weight of the sample in gram X 10,000 Area of sample in cm2

The higher the GSM, the higher the thickness and weight of the fabric will be. GSM is an indication of the quality of the fabric too.