Initialism of gender or sexual minority. (Compare GSRM.)
"The printer shop ordered reams of cardstock rated at 300 gsm to ensure they were thick enough for business cards."
Initialism of grams per square metre (unit for measuring the weight of paper)
In plain English: GSM is an international standard that sets rules for how mobile phones work and connect to networks around the world.
"The new phone has better GSM signal than my old one."
Initialism of Global System for Mobile communications.
"The new smartphone operates on a GSM network that covers most of Europe and Asia."