What is the definition of GSM when looking for specific fabric?

What is the definition of GSM when looking for specific fabric?   You’ve heard of a 50/50 or a 80/20 t-shirt. 50/50 means 50% cotton and 50% polyester or 80% cotton and 20% polyester, but have you ever seen the gsm listed and wondered what that meant? Simply put; that’s the weight of the fabric.