If you look on Steam, the Windows version of the game is up to version 1.3.5. My Linux version is 1.3.1b.

When can we expect the Linux version to be updated?

Yes, apparently Steam updated the game yesterday. Why were you looking for the game on Steam, anyway? When it's ready. Either you can exercise a little patience, or get the game on Steam. Devs generally prioritize Steam, as you should well know. Besides, some of us don't need all our games updated every other week. What specific change are you desperately waiting for?

Galaxy just updated from 1.3.1b to 1.4.0 on Windows, so I'd assume the 1.4.0 installers / patch should be coming to the website today.

Yes, I should not have answered you to begin with. Normally I ignore these posts by people seeing updates on some other site and wondering when it will finally get to gog. We get way too many of these, and I don't care if you were "just asking" or being an impatient twit, since I can't tell the difference. If you weren't concerned about the Sunday release on Steam the day before posting, you wouldn't have mentioned it. I only replied because you said "Linux", and for some reason, I still look to see if I can help Linux users, even though I hate everybody, and do not benefit in the slightest. I did actually answer you, if you cared to read. The policy is, as determined by the publishers/devs, not gog, to update gog whenever they feel like ("when it's ready"). While there is often no alternate policy for Linux, as Kalnyr pointed out, Windows and Mac users may sometimes get preferential treatment due to the lack of Galaxy for Linux, and gog's piss-poor Linux support in general. Supposedly gog tests patches sometimes before release, but given that we still get hot fixes and broken releases (especially for Linux; I doubt they test that at all), I'm not sure that does anything but delay patches.

IN any case, non-Galaxy 1.4.0 installers released today. We won't be getting those interim releases..

As version 1.4.0-39735 is there, it may get calmer now.
I like UPDATES coming in - but if all updates would come at least 4 weeks after the release I would be happy - but of cause the faster the better for all gamers.
And as experience shows asking for missing updates can help from time to time - so doing this (maybe a few days after the release) may be helpful - and not like a child annoying everyone. So this HAS a reason.
I would use other words to describe it - but I think the difference between user expectations concerning SUPPORT and what GOG uses the word for ... (which even varies a lot - supporting LTS only and having another flavour of support extended on STS where all just do hope it may work - cough ... similar to what DRM-free means - which may be more problematic when knowing something about RMS and GNU/FSF/GPL; or even something easy - at first glance - like controller support ...).
But the problem is not only to blame on GOG - as many devs seem to have more experience with other platforms ... so one should help in that respect.
And GOG appreciating help from gamers by supporting them in that help would be very nice ... someday ... in the future.
Still waiting ... like downloading development branch without Galaxy (we have a download page, so why not putting that there too?) - with that client being a no-go anyway for those who care about the DRM-free thing ...
And also a yes about LACK OF TESTING / QA - one should note when a game has no 64 bit binary when it should have one and work on getting it - and not let gamers search for it and not answering about bringing this to a normal install script (concerning another game of cause).
But one can be more optimistic in saying:
     GOG has really a lot of potential to increase their Linux support. ;)

lgogdownloader already supports Galaxy downloads, as far as I know, including for Linux installers by reading/installing from the zip file attached to the installer binary. If Galaxy were supported for Linux, it would bring two benefits, even for those, like me, who would never use it (yes, for pretty much the same reasons I would never use Steam): the availability of Galaxy-only downloads (via lgogdownloader or a similar open source tool) and the release of Linux binaries by devs who refuse to release because of the lack of Steam-like libraries for Linux. As is, given the screaming by Windows users for achievement support, if Galaxy ever does release for Linux, we'll hear a similar screaming about games that aren't even supported by the developer any more due to the fact that gog didn't even bother to provide dummy libraries that could be swapped out if/when the real ones ever came to being. The same goes for multiplayer matchmaker services & the like.