Subtract.
Gross is the amount you started with. Net is the amount you have left after some was taken away. (for whatever reason)
So if you gross $1000 and than pay $200 in taxes you "net" $800
1000 - 200 = 800
If you gross $1000 in sales volume but it cost you $900 to produce the items you sold your "net" would be $100 profit
$1000 - 900 = 100
Ninjavega