So this is an open source game, being sold for $8 on GoG for the convenience of having someone compile it for you?

Am I missing something?

You're paying for convenience, a few cosmetic extras, and to support me so that I can keep working on updates for the game.

Here is the actual game this is based on:
https://watabou.itch.io/pixel-dungeon

Available for free there for Windows, Linux and Mac PCs, as well as mobile devices (links for the relevant mobile store pages are on the site).

You can think of it as a demo of Shattered Pixel Dungeon seeing how this one is adding features on top of existing ones.

Got it, thanks. Just making sure I understood everything correctly.