GSM — grams per square metre — is the single biggest lever you have over how a printed piece feels in someone's hand, and how much it costs to produce.
A quick reference for common jobs
Most first-time print buyers focus on colour and finish, and treat paper weight as an afterthought. In practice, GSM affects perceived quality, postage cost, and how well a piece survives handling — sometimes more than the printing technique itself.
As a starting point: 100–130 GSM suits flyers and single-sheet handouts that are read once and discarded. 170–250 GSM is the sweet spot for brochures, folders and posters where some rigidity is expected. Business cards typically start at 300 GSM and go up to 450 GSM for a noticeably premium, rigid feel.
Packaging is a different category entirely — folding cartons are usually specified in board thickness (microns or point size) rather than GSM, since structural rigidity matters more than sheet weight.