HunchBluntley: I just checked my download log, and sure enough, when I downloaded it earlier, I downloaded the Linux build without noticing. Well, at least it's a small file. XD
...Wait, no it's fucking not. Why is the Linux download for the editor 1.5 MB, but the Windows version is 418 MB?! Is the modkit download under DLC goodies just for use with the Linux version, while the Windows version already includes it?
Caban: I had windows version selected anyway but you are right, I switched it to Linux and then back to Windows and now I see proper installer on second pc too, thx. :) Maybe something was wrong in cookies, happens.
However it's strange that Linux has such efficient decompressing abilities to make 417mb out of 1mb. ^^ I guess someone should take a look at it. :P
Mortius1: As far as the dramatically different file sizes are concerned, I expect that the Linux version of the modkit requires the game to be installed and share's the game assets. On the windows side these get duplicated so you end up with two sets of models, textures etc.
That being said, I haven't actually installed the modkits to confirm this.
The modkit is only for Windows, so likely GOG's UI can't handle this properly and generates a stub file.