A common misconception is that the letters GSM on boxes of inkjet paper refer to the thickness of the paper. This of course is not true.
GSM or g/m2 stands for Grams per Square Meter and is a measure of weight not thickness. Simply stated, if you were to pour one square meter of the pulp of the paper on to a flat surface, how many grams it weighs equals the gsm.
Thickness is measured in mils or microns. A mil = 1/1000th of an inch and a Micron = 1/1000th of a millimeter.
GSM, being a measurement of weight of course, means that many papers of a certain weight, let's use 310 gsm as an example, can be different thicknesses based on the density of the pulp used in manufacturing the paper. There are RC papers that are 310 gsm that are stiff but thin compared to 100% cotton fine art papers which feel thick but more spongy.
Bottom line is that not all papers of the same gsm are the same thickness. Asking for a 310 gsm thick paper is the wrong question. You really need to physically touch the paper, or as I say, "allow your fingers to taste the paper." Your fingers will send electrical impulses to your brain letting you know whether or not you like it or not. Photography is a multi-sensory medium and as an artist, viewing a print with your eyes is only one part of the process. We make many decisions based on how things feel. Inkjet paper is the same way,