What does GSM mean in paper?

That distinction catches people out. Two boards at the same GSM can have noticeably different thickness, because one is denser than the other. A bulky, uncoated board and a compressed, coated one can weigh identically per square metre and behave completely differently in a die. When the physical thickness is what matters — and on packaging it usually is, because it drives the dieline — the measurement you want is caliper.

As a rough orientation: office paper is around 80 gsm; a decent letterhead 100 to 120; a brochure cover 200 to 300; a business card 300 to 400. Folding cartons typically start around 250 gsm and run up past 400 for larger or heavier packs. Corrugated is described by flute and liner weights rather than by a single GSM figure.

Choosing GSM is a balance rather than a maximum. Heavier stock feels more substantial and resists handling better, but it costs more per sheet, adds shipping weight on every unit, needs wider creases, and past a point becomes harder to fold cleanly. Going up a grade "to be safe" can produce a carton that cracks on the fold and costs more to post.

For any packaging job the sensible route is to prove it on a physical sample in the actual board rather than to argue about the number. Rigidity is easy to feel and hard to specify.