Actually this is kind of tricky, 1oz. does equal 28.95g., but 1 meter does not equal 1 yard....GSM is grams per square meter
So if a shirt is 5oz it is 142 gsm
Basically 1 oz is equal to 28.35 grams and that is how you get your calculation
If someone says they have a 200gsm and you want to know the oz just divide by 28.35 which is 7 oz
Hope this helps