GSM measures fabric weight per square metre. Lightweight fabrics run 50-150 gsm (chiffon 50, silk 90, jersey 180). Mid-weight 150-300 gsm (cotton 160, linen 180, wool 280). Heavy fabrics exceed 300 gsm (canvas 380, denim 340, tweed 350). GSM directly affects drape, warmth, and packability. Low-GSM fabrics pack tightly and dry fast. High-GSM fabrics provide structure and insulation. Climate matching: hot-humid contexts favour sub-200 gsm; cold contexts need 250+.