GSM stands for grams per square meter and refers to the weight of the fleece. There are typically three major categories, but a fourth has been included to really narrow things down. A 280gsm fleece will fall into the midweight range as it is ideal in cool to cold conditions and can be worn alone,as a midlayer, or underneath a shell. Think of fall and winter adventures on the trail or on the slopes for this fleece layer.
Ultralight - less than 100gsm
Lightweight - 100-200gsm
Midweight - 200-300gsm
Heavyweight - 300gsm or more