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https://www.gog.com/promo/20191206_promo_launch_daymare_1998
NOTE: If you own Shovel Knight: Treasure Trove, you will NOT need to purchase those new Shovel Knight game modes separately.

https://www.gog.com/promo/20191211_launch_promo_shovel_knight_expansion

GOG have an official announcement for those new game modes, but said nothing about the promo.
https://www.gog.com/promo/20191212_winter_sale_focus