I've just learned of this term but it is perhaps nothing new to many here. m-w.com turns up zip on this, dictionary.com too. The term is apparently fairly well known amongst publishers though. It is defined here www.gpcpapers.com/glossary.cfm as follows:

"The mass of a unit area of paper or board determined by the standard method of test: it is expressed in g/m2."

So is it etymologically related to 'grammar', and does this mean that correct writing carries more weight?