Just to throw this out in case another user goes looking, a staff member clarified that you either have to add to your wallet funds or make a direct purchase to qualify for the free game:
https://www.gog.com/forum/general/the_black_friday_sale_2017_begins_3094e/post243
Unfortunately, wallet funds spent on your purchases won't count towards your stretch goal. If you buy wallet funds, this transaction counts towards the stretch goal. It's all about money being spent.
Hope that clears it up.
Using current wallet funds won't count against the $15 requirement.
Hope this helps