I contacted GOG a year ago and asked for versions numbers for some games:
* Ziggurat - patch 14, from June 2016
* Lucius - 1.02.3866 [1.043
on Steam]
* Nuclear Throne - 58883919 / 01-12-16 (our version, without Steam functionalities like leaderboards)
* Outlast & Outlast: Whistleblower - 40474204
* Ori and the Blind Forest: Definitive Edition - GOG-2 / 5.3.2 / 9582331
* Stasis - patch 1.09
* Trine 3 - 1.11 build 3102
* Magrunner: Dark Pulse - 26828389
* The Testament of Sherlock Holmes - there's no game version number, only our internal GOG-5
* Pathologic Classic HD - patch 1.0.3
* Deadly Premonition: The Director’s Cut - 1.01b
* Postal 2 - US 1415 Hotfix2
* Postal - doesn't have the specific number as well, but it's the latest one for sure [is it?]
* Star Wars Knights of the Old Republic II: The Sith Lords - 2.1.0
The list is too old to be used as a source, but it at least confirms that games such as Deadly Premonition and Pathologic are up to date.