GSM is the metric way to state fabric weight. US blank makers usually quote ounces per square yard instead; one oz/yd² equals about 33.9 GSM, so a 6 oz/yd² tee is roughly 203 GSM. Brands do not all measure at the same point or in the same way, which is one reason two shirts with the same listed weight can feel different.
Weight hints at thickness, drape, opacity and durability, but it is not the whole story. Yarn type and size, knit structure and finishing all change the feel. A tight knit of fine ring-spun yarn can feel smoother and more substantial than a looser open-end shirt of the same GSM.
For merch, GSM helps match a blank to its wearer: lighter fabric for hot climates and events, heavier for a premium streetwear feel and a stable surface under big prints. Heavier garments also cost more to ship. When comparing styles, check which unit each maker uses, then read the GSM guide and the weights guide.