GSM is grams per square meter. That's it. It's the weight of one square meter of the fabric, and it's the single most useful number on a spec sheet because it's the hardest one to fake.
A cheap promotional tee runs 140–160gsm. You can see your hand through it. It'll shrink a size, twist at the side seam, and the collar will be wavy inside a month. A mid-tier retail tee is 180–200gsm. Fine. Forgettable.
We build at 240gsm. It drapes instead of clinging, it holds a printed graphic without the ink cracking, and it survives a hot wash because there's enough fabric there to survive one.
Fleece is a different scale. A mall hoodie is 260–300gsm. Ours is 380. The difference you feel immediately is that it has structure — it holds its shape off your body instead of collapsing into a puddle on the shop floor.
There's a ceiling. Past about 420gsm a hoodie stops being wearable indoors and starts being outerwear you take off. 380 is the line where it's still a hoodie but you'd trust it in November.
The catch: heavier fabric costs more per unit and ships heavier. That's the entire reason most brands don't do it. It's not a manufacturing secret, it's a margin decision.