I tried downloading the Windows Installers of "Tower of Guns" and "Seven Kingdom 2 HD" through the GOG downloader and got "Checksum error on chunk 1" for both.
Tried a couple of other games (Capsized, Capitalism Plus) and had no problem.
Anybody else sees this?

Thanks for the clarification.
Anyway, I think I may have jumped the gun a little with my indignation. I hadn't yet read any of today's release/preorder threads when I made my previous post, and so when I saw someone in this thread mention "first region-restricted game on GOG", my first thought (in light of the recent addition of German ratings to various gamecards) was, "Great, GOG's laying the groundwork for the launch of German language support by adding ratings to, and probably region-locking or -censoring, certain games." I wasn't even thinking of Australia! ...Still, this does set a very bad precedent, and I wouldn't be surprised now if broader region-based restrictions are eventually put in place. I hope it doesn't come to that...but if I lived in Germany or Australia (or anywhere else with similar censorship laws), and had any interest whatsoever in any of the titles GOG sells which are otherwise either censored or unavailable in those places, right about now I'd be looking at buying those games and downloading them ASAP.

From MaGog's logs for 26 February:

NOTE! CHANGED Republique Remastered [Upcoming], bonuses: ***TO*** manual (coming soon)

I just downloaded Seven Kingdoms 2 HD with the downloader no prob. Try exiting it, restarting, redownloading. If that doesn't work, contact GOG support for help, perhaps you're experiencing an odd bug or something in the downloader.

No problems with SK2 HD here, downloaded without a single problem.

Thank you. Before posting I had already tried all your suggestions, plus rebooting my computer, without avail.
However, I tried again just now and Seven Kingdoms downloaded just fine. Tower of Guns still fails.
It is my experience that posting in this thread often solves issues faster than contacting support.
Anyhow, I'm not too troubled by it, as I don't use the downloader on a regular basis. I download via the browser.
Will wait and see.

Yeah, it's gone again from my account too.

Still there on mine.

Mine's gone!
Just Fallout 1 :)

While I am grateful for the quick response I would appreciate if you could answer a question. Are you guys working on finding and fixing the problem that is causing this game disappearences? Because in the last months things like this have happened quite often and it really gets annoying if you can trust your own account anymore to have all games and needing to constantly check if everything is still there. Thanks.

Mine's there too.

The fixing will break something else, so the best we all can do is to back up our installers elsewhere. =/

My Fallout 1 is gone too :-/

can confirm, Fallout 1 gone, 2 & tactics are still there.