I live! ... just kidding, I don't. Braaaaainsss~
Er, anyway, the game looks interesting. I enjoy rogue-likes as well as platformer games, so it could be a fun combo. Does anyone know if this game has permanent unlockables, similar to Rogue Legacy?
As for extras, personally I enjoy seeing concept art, design sketches and such, being an artist myself. Other than that, I think wallpapers and avatars should be standard extras for every game. Free soundtracks are also great, when available. Beyond that, I'm not sure what GOG or the developer could add to their games. Some walkthrough or hint files would be convenient, so people don't have to search the web when they get stuck.
Overall I'm disappointed with the lack of decent extras for many Indie titles on here. Many of the basic extras I mentioned would take ( almost ) no additional effort at all. I suspect the developers and/or publishers are to blame for this, not GOG. Maybe they're not used to this whole "extras" business, since Steam doesn't seem to bother with it.