GSM stands for grams per square metre. If you cut out one square metre of a fabric and put it on a scale, the number you get is its GSM. A 280 GSM fabric weighs 280 grams per square metre. That is the entire definition. It measures the weight of the fabric, and nothing else.
It is worth knowing because it is one of the few objective numbers you ever get about a T-shirt. Product photos cannot show you weight. Words like heavyweight mean slightly different things wherever you read them. The number is just the number, and you can compare it across anything that publishes it.
DROP 001 uses 280 GSM fabric and we describe it as heavyweight. You notice that weight when you pick the garment up and when you wear it. The number tells you how much the fabric weighs. It does not tell you whether the T-shirt is better.
This is the part worth being honest about. The number says nothing about the cotton itself. Fibres can be long or short, cleanly prepared or not. DROP 001 uses combed cotton, which means the shorter fibres are removed before the yarn is spun, and what is left makes a smoother, cleaner surface. Two fabrics can both weigh 280 grams per square metre and feel nothing alike. The number also says nothing about the knit, the cut of the garment or how well it is put together. And higher is not automatically better. A heavy fabric in the wrong garment is just heavy, and for plenty of situations a lighter fabric is the more sensible choice.
So treat GSM as one honest input, not a verdict.
One common mix-up is worth clearing. GSM is weight, not thickness, even though the two usually travel together. How thick and how stiff a fabric feels also depends on how tightly it is knitted and what yarn it is made from. A tightly knitted fabric can feel denser than a looser one at the same weight. So when a shirt is described as thick, that is an impression. When it is described as 280 GSM, that is a measurement. Useful to know which one you are being given.
Why does DROP 001 use 280? Because of what this particular garment asks the fabric to do. The cut is oversized and boxy, and volume like that needs weight to hang properly instead of folding in on itself. The colour is seedpearl, a warm off-white, so the fabric has to stay solid in daylight on its own. The garment also uses a raised puff print on the back. Using more fabric affects the cost, but the number was never the point. The behaviour was the point, and 280 GSM is where this garment worked.
One practical note. Heavier cotton is straightforward to live with, but it is worth washing properly. Cold, at 30°C maximum, inside out, no tumble dryer, dried flat. The care guide has the full list and takes under a minute to read.
If you are comparing T-shirts online, look for the GSM. Not because a bigger number wins, but because it is one of the few facts you can actually compare. Then judge everything else from the garment itself.