Whether gst calculates before or after a discount is something  you need to check on with the your government offic

if the item is taxable, and the discount is also taxable it works like

100 T
-20  T

80  T

gst will then be calculated on 80

but, if the discount is non taxable

100

+gst

-20

if you are getting less than 80 including gst, that is weird