On each achievement edit screen you will see a new box with the title “Award a discount”. Just checking the option “Award users that earn this achievement with a discount?” you will be able to configure the discount that will be served when the achievement gets completed.

Discount Type
Type of this discount. Available options are: Fixed Price and Percentage Price.

Amount
Amount of discount to apply. If you set Discount Type as Fixed Price this amount will be applied as a flat amount. If you set Discount Type as Percentage Price this amount will be applied as a percentage. In both cases, you do not need to enter any symbol (currency or %).

Apply to all downloads
Checking this option will apply the discount to all downloads.

Note: You can exclude downloads through the “Downloads Excluded”.

Downloads (Only if Apply to all downloads is not checked)
Downloads where discounts will be applied.

Downloads Excluded (Only if Apply to all downloads is checked)
Downloads excluded to be discounted.

Maximum uses
Maximum number of uses (set 0 for no maximum).

Maximum uses per user
Maximum number of uses per user (set 0 for no maximum).

Label
Label to show on fees.

WooCommerce Discounts adds the paratemer downloads_discounts to the [gamipress_achievement] shortcode which accepts yes and no as value. By default this parameters is setup as yes.

Example:
[gamipress_achievement downloads_discounts="yes"]

Also, on the GamiPress: Achievement widget will appear the Show Discounts option to let you enable or disable it.

Both methods will output a list of the discounted products as well as the discount amount: