spindown: People posting these code breaking challenges always underestimate how difficult it can be to solve them. (...) What seems obvious to you is not obvious at all to others, and the possibilities are endless.
Indeed, which is why I try to apply some base logic to the puzzles I provide.
If one's aware that gift codes on GOG consist of [A-Z][0-9], you can basically start trying to project (and I never really randomize my mapping but go straight A to Z for the very reason of solving without much brute force).
I confess though that my wicked mind might betray me at times.
Lexor: full three octaves
I would have preferred this, but couldn't find an application that provided me with the possibility for free ;)
I thought that this set-up would be okay though as it was displayed visual and the amount of possible semitones was somewhat highlighted.
GameRager: If you don't know music it is hard.
That's the great part about this riddle. You don't have to know anything about music to solve it. The tricky part might actually have been getting away from the idea of music and it's relevance!