What does ‘gsm’ mean?

Grams per square metre. It’s a measure of paper weight. Higher gsm means thicker, heavier stock. Some stocks have a high gsm but aren’t thicker, board is also measured in microns, literally thickness. A high gsm but low micron means a really compact paper pulp so will be rigid. A low gsm but high micron would be something really pulpy (think cereal box) we try to strike a balance between weight and thickness.