How many millimeters are there in 300 gram per square meter?

Answer 1

None.

Grams per square metre is a measure of planar density while a millimetre is a measure of distance (or length). The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.

Answer 2

There are 300 grams per square meter, which means there are 300 grams spread over an area of one square meter. To convert this to millimeters, you would need to know the thickness or height of the material in question (e.g. paper, fabric) to calculate how many millimeters this weight would cover.