A Linux version of D:OS was confirmed when the kickstarter was funded. After that, Larian said that since the same engine was being used for both games, that would mean a Linux version of Dragon Commander (which sounded probable, if not definite). More recently, didn't someone say the Linux version would have its own beta? (maybe Swen in one of the twitch.tv Q&A sessions?)
As for GOG, Larian and GOG both know there will be Linux versions of the games, so there is likely a plan in place. However, we're unlikely to get an official comment until GOG actually announces Linux support or the game is released and some alternate arrangements (Larian Vault) are made. I would suspect GOG wouldn't want to add it as an extra if they don't have official Linux support by then, if only because that would add a ton of extra bandwidth for people using the GOG downloader to get all the extras without paying attention to the size.
