GSM stands for grams per square metre — the weight of pile material packed into each square metre of a rug. It's a density measurement, not a size or thickness measurement, and it's one of the more reliable numbers for predicting how a rug will actually perform, as opposed to how it photographs.
A higher GSM means more fibre packed into the same area, which generally means a rug resists crushing and matting better and holds its shape longer under foot traffic. Two rugs can look identical in a photo and have very different densities — pile height alone doesn't tell you this, since a tall, loosely packed pile can actually feel less substantial underfoot than a shorter, denser one.
It's not the only variable that matters, though. Construction technique changes what density even means: a hand-knotted rug like Rosewood Medallion is inherently dense by nature of the knotting process itself, while a hand-tufted rug's density depends more directly on how tightly the tufting gun packs each row. Material plays a role too — 100% New Zealand wool holds density and bounce-back better over time than many blended or synthetic fibres at an equivalent weight.
In practice, density is why a rug in a high-traffic hallway or under a dining table should generally lean toward a denser, lower-pile construction, while a plush, lower-density piece is better suited to a bedroom or a spot with lighter use. It's a genuinely useful spec to ask about — more useful, honestly, than most of the marketing language rugs get sold with.