GSM is grams per square metre — a measure of mass, nothing more. It is useful for comparing two blankets of the same construction and misleading for anything else.
Warmth comes from trapped air, and structure decides how much air a fabric holds. Our 320 GSM waffle is warmer than a flat 380 GSM twill because each cell holds a pocket of still air.
For bedsheets, GSM tells you about hand and durability. 130 GSM percale feels crisp and light; 170 GSM jacquard drapes heavily. Neither is better — they are different beds.
If you buy for a hotel, ask for weight loss after ten industrial washes. That one figure predicts a piece’s second year better than any label.