What GSM makes a good T-shirt?

GSM — grams per square meter — is the fabric weight, and it is the first number to check. Under about 150 GSM is lightweight and often a little sheer; 180–200 is a solid everyday weight; 220 and up is a true "heavyweight" tee that holds its shape and drapes with structure. Heavier is not automatically better — it is warmer and less breathable — but it usually wears longer, which is where the cost-per-wear case lives. The real tell is whether a brand publishes the number at all. When GSM is missing from the spec, weight is exactly the variable being left vague.