The letters gsm stand, in an unscientific way, for the words grams per square metre.
A gram is a unit of weight or mass, equal to that of one fifth of a UK twenty pence piece ~ which weighs exactly five grams.
A square metre is an area equivalent to a one metre by one metre square of plane material, such as paper or cloth, or the surface of the ground. An area of one square metre does not have to be square in shape ~ it can be triangular, rectangular ~ circular ~~. The floor area of a living room might be 15 square metres ~ a doubles tennis court has a surface area of 261 square metres ~ football pitches vary somewhat in size (tennis courts do not) but the preferred area is 105x68 metres, which is 7,140 square metres (or 0.1 hectare).
A square metre of paper is very large.
The distinction is subtle, but is worth mentioning ~ a metre square is also a square metre, but that wording is used to indicate that the material is also in an exactly square shape.
gsm is most commonly used as a shorthand way of describing the substance of paper, or cloth. There is no point in weighing a piece of paper unless ou specify its size. 'This sheet of paper weighs 5gm' is meaningless. 'This sheet of A4 paper weighs 5gm' is helpful in describing one of the properties of that paper. Most sheets of A4 office paper weigh 5gm. One sheet of a better quality paper weighs about 6 or even 7gm.
The substance of a paper is typically given in its weight per square metre, although for some purposes (artists' papers) and some countries (USA) different indications are used. A square metre sheet of office paper typically weighs 80 grams. 100 gram, or 100gm, is normal for better quality paper, and 120gsm might be used by a prestigious firm, wishing to exemplify quality-in-everything-we-do.
A quick way of checking the substance of a paper would be to weigh a metre square, or a square metre, or the equivalent amount cut into more convenient smaller pieces. It so happens that a sheet of A4 paper is, by definition, one-sixteenth of a squaremetre. So 16 sheets of A4 paper will weigh as much a one square metre would. Weigh 16 sheets of A4 paper ('postal scales' will do this well ~ kitchen scales might not be sensitive enough) and the weight gives you the substance of the paper.
