I'm not sure I agree. "GSM" is three syllables, versus four for "grammes per square metre". You can write it correctly using only characters everyone knows how to type quickly on their keyboard, versus either finding a way to get that superscript ² or else typing something like g/m^2 which is uglier and longer. And you can use it comfortably even if you are a complete mathematical ignoramus (you just need to know things like "larger numbers mean heavier paper" and "cheap printer paper is about 80gsm" and so forth) without the risk of turning g/m² into the nonsensical g/m2 or something.
(But arguably what whoever decided on "gsm" should have done was to just use "g", with the "per square metre" left implicit.)
I beg to differ. You can totally get away with g/m2 which is not hard to type and crucially has a / to hint you what it could be about
"gsm", or even more so "GSM", belongs to the reign of abbrevations and put my brain on the wrong track
Roughly no one already says GSM. When talking about paper you'll hear people say things like "That's a sheet of 120 gram"
GSM basically only ever appears in print. If someone DOES ask "what does 120 gram mean here?" the clarification is going to be "Oh that's grams per square meter" and not "Oh that's gee es em"
I should mention GSM is also probably an americanism. I'm in the EU and out of the five packs of different kinds of art paper four are labeled in g/m2, and one has no labeled weight at all. None of them are marked in GSM as that abbreviation only works in english, while g/m2 works in all languages.