Short answer
GSM is grams per square metre, a measure of fabric weight; width is how wide the roll is, commonly 44, 58 or 60 inches in Surat. Together they decide how much fabric you get for your money.
Two fabrics with the same name can differ in weight and width. A heavier GSM usually means a denser, more opaque fabric. A wider roll means more garments per metre. Compare quotes on price per square metre, not just per metre.
Always put composition, GSM, width and finish in writing, and measure the width of the sample yourself.
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