AngryHatter: What version does the developer offer?
This doesn't clear it up...from his website.
" Buyers of the game direct from us (the developers) get DRM-free download links to PC, Mac and Linux builds of the game, as well as a 'steam-key' to redeem on steam."
Seems pretty clear to me. You get download links for Windows, Mac and Linux versions and a Steam key if you prefer to use Steam. It clearly says 'DRM-free', as you quoted. Also: the developer has his e-mail address on the website, so you can email him for questions and answers ... (hint: click on 'BUY NOW!', scroll down and find: 'problems ordering? email cliff').
This forum is probably the last place I'd look for definitive answers. ;) The first place to look for information on anything should always be the website of the developer.
[edit:] That said: I bought it off of the developer's (Positech co uk) website and got all the versions (Windows, Mac and Linux) to download without DRM. And like I said it is working on my Linux machine just fine. It's a native Linux binary (ELF 32-bit LSB).
I honestly don't understand why gog.com refuses to offer native Linux builds.