What does GSM mean?
GSM means grams per square metre. It describes the weight of the paper, not necessarily how thick or stiff it feels.
Two papers with the same GSM can feel very different depending on coating, bulk and fibre.
A practical guide
80gsm to 100gsm is common for letters and office stationery. 120gsm to 170gsm suits leaflets and booklet text. 200gsm to 250gsm works well for covers and postcards. 300gsm to 400gsm is often used for business cards and invitations.
Heavier is not always better
The right paper depends on how the item will be folded, posted, handled, written on and stored.
A heavier sheet can create cracking, folding or postage problems if it is chosen simply because it feels more expensive.
Ask for samples
Paper is tactile. Where possible, handle a sample before committing to a large run.