I don't know what Great Weapon Mastery does, but rerolling is pretty easy. Assuming you can input whatever damage calculation you need, you either add r<2 (reroll all rolls that are 2 or below) or ro<2 (reroll once and take the second result even if it is 2 or below). So your damage might look like 2d6ro<2. <a href="https://wiki.roll20.net/Dice_Reference#Rerolling_Dice" rel="nofollow">https://wiki.roll20.net/Dice_Reference#Rerolling_Dice</a>