GSM stands for Grams per Square Meter, also called grammage. It's the metric system's way of measuring paper weight and thickness.
The beauty of GSM is that it's consistent. It doesn't matter what size the sheet is. You're always measuring the weight of one square meter. So 200 GSM paper is always 200 GSM, whether it's a text stock or a cover stock.
This is different from the US pound system, where the basis sheet size changes depending on the paper category. That's why 80 lb text and 80 lb cover are completely different thicknesses.
For reference: standard copy paper is about 75 to 80 GSM. Cardstock is typically 200 to 350 GSM. Watercolor paper is 300 to 640 GSM.