Heavyweight tees hold their shape and block light. Lightweight tees breathe better and move with your body. The right pick comes down to your climate and how you plan to wear the shirt. What Makes a T-Shirt Heavyweight or Lightweight? GSM stands for grams per square meter, the number that shows how much cotton sits in the fabric. A heavyweight black and white t-shirt usually falls between 200 and 260 GSM. A lightweight one sits closer to 120 to 160 GSM. Higher GSM means thick