You can however grab a GOG key from the Humble Widgets on our
buy page. They're pretty close in what percentage of the sale goes to us.
Sadly it does not seem that Heart of Gold and The Under Games expansions are available without buying the base game again through Humble…
Well, I can wait for the options to expand in some ways, I still need to complete the original campaign + My Pet Dungeon scenarios (my favourite game mode). I would love to say the same about the Crucible game mode, but right now it is far too hard for my limited skills.
Maybe I should ask GOG support too if they would be OK to turn the base game, the Underlord Edition additions and My Pet Dungeon to gift keys if I buy the full Ultimate Edition already owning these.
I know a couple friends who would be happy with such gifts
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EDIT: Oops, looks like I just missed them, both expansions are actually available on the Humble store, there are just no widgets for them on your website.