The first thing Galaxy does when you try to start a game is to synchronize the cloud saves, so I suspect there was probably a bug with said cloud saves that was corrected in the latest updates.

I just installed Gothic 2 and the shortcuts added to the start menu launch the game using Galaxy so I did not click on them. Instead, with Galaxy not running, I went into the game's directory and launched the game using the shortcut found there, which just invokes the game executable.

While doing this I had wireshark monitoring network traffic, as well as my firewall blocking all known *.gog.com domains and logging any attempted traffic (Linux netfilter). My results are that nothing showed up in the logs in terms of attempts to connect to GOG.com, and there was no other network activity at all whatsoever caused by the game executable. Note that when launching games in Windows it will cause Windows itself to attempt to connect to Microsoft to spy on your game playing, at IP address 65.55.162.26 but that is part of Windows and nothing to do with any game. I do not count that traffic (and I block that at my border firewall as well.)

I do however have to report that the game executable did start up and remain running, but in the background. It never initialized any form of graphical user interface nor go fullscreen, it just started up into the background and sits there doing nothing as a broken unusable game so far. Killing the game with process manager and restarting it by using the shortcut results in the exact same results every time, a non-working game.

I've killed the dead game and am now I am firing up Galaxy client to try and launch the game...

Ok, that's wonderful. If Galaxy is launched and there is no network availability (remember I firewalled it using REJECT with icmp-adm-prohibited, so any network traffic should be notified that the host is unavailable and do the appropriate thing and bail out), then Galaxy will never bring up its user interface. So I'm not really sure how using "Galaxy offline" is supposed to work if the user interface never comes up if it can't connect to GOG's servers. Perhaps they have never tested Galaxy client without a working network, hard to say.

Looking in my process list I see GalaxyUpdater.exe running but no other Galaxy related processes.

<3 minutes pass> Ok, I got a dialog box popup informing me "Galaxy update problem". This is pretty poor user experience IMHO if someone does not have a working network and just wants to run Galaxy and launch a game offline. The client should display something visible to the user FIRST, before any network activity, and give some indication that it is at least attempting to do something. Under no circumstances should any application freeze in the background for 3 minutes with no user visible indicators or user interface just because the network is unavailable. Kind of disappointing.

So, I click on the [OK] button and am told to gg download the latest version of Galaxy (which I already have), and then it exits and I have no running Galaxy. Awesome that.

Another entire minute passes and Galaxy client actually comes up now and appears to be ready to use with a big "NOT CONNECTED" button at the bottom of the screen. I dunno if anyone from GOG will be reading this or not, but this is just poor user experience having to wait 5 minutes to get a usable client screen simply because the network is down. Please fix this. No user should ever have to wait more than 10 seconds or so for the client to come up and be able to launch already installed games whether or not a network cable is even plugged into the machine.

Anyhow... I launched Gothic II via Galaxy now, with the network still disabled. The game executable started and went into the background with no visible indication that it launched, and no recorded network traffic detected other than Galaxy attempting to do what it always does trying to connect to GOG, but that has nothing to do with the game itself. Final results: Gothic 2 does not work.

And now for the final test... I killed both Gothic2 processes and all Galaxy related processes, I've re-enabling network access to *.gog.com and related subresources, and I've relaunched Galaxy client. I've confirmed that Galaxy is able to communicate with GOG's various servers, CDNs, subresources etc. Launching Gothic 2 does not trigger any network traffic here, and once again the game executable starts up into the background and the game never shows its face to me, dead in the background.

Final conclusions:
1) Gothic 2 does not generate any network traffic for me personally whether or not Galaxy is being used to launch it.

2) Gothic 2 does not launch and run at all, the game is broken for me at least and probably requires scouring the game specific forums here and googling around to find game bug workarounds or whatnot. If that does not turn up a solution, I'll contact GOG support as they are miracle workers when it comes to getting these games to work under the worst of conditions.support.

3) Galaxy definitely needs some user friendliness improvements in terms of showing a user interface and some kind of status and interaction with the user within 10 seconds or so of launching regardless of whether or not the program is able to connect to any online resources. There should never be a 3-4 minute delay before a program shows its user interface to a user simply because the network is down. That's just poor design IMHO, but it's nothing that GOG's talented developers can't fix. I might file a bug report in mantis for this some time in the future, but only when I have time to put together a proper fully researched bug report with enough detail to be fully useful. Not a priority at the moment though.

So there ya have it, my results of trying to run Gothic 2 in every way imaginable -> FAIL. Just some datapoints to add to the pool.

That is the Games Explorer or whatever it was called at fault. The first time you run the game, it tries to identify it and find data for it, which may fail (and you don't get a message about it unfortunately). If you run the game again while the first process is running though, it will launch normally, and you can then kill the not responding process.
I don't recall, but I do think that it wasn't needed to do that again.

I just tried it by disabling my network adapter. Galaxy launched immediately (or at least as fast as I recall it launching otherwise). Beefy machine, but as I recall, so was yours.
It is possible that your machine does an internet connectivity test that you do not block, gets a reply, thus tries to get a version check which is rejected instead of giving the expected code. Do try the test again with a properly offline machine instead of a "Let's reject what we think needs rejecting" and see if the Galaxy experience is the same or not. Feel free to ask if you need me to duplicate your tests as well.

Interestingly, I had a similar experience with Steam just today. Earlier today I started Steam, the login window came, I gave my Steam password... and then it just seemed to freeze there, not going anywhere. I waited for a minute or two and then hit cancel, and Steam exited. I tried launching Steam again, and exactly the same.

Only then I noticed an exclamation mark on my network connection icon, meaning there was some connectivity problem with my WLAN router (it occasionally does that, then I need to reset the router). I fixed my internets, and after that Steam worked fine (proceeded after the log in).

So to me it seems Galaxy and Steam have similar oddities when the network is down. I recall earlier that if the internet connection was completely disabled (LAN cable disconnected and no wifi enabled), then Steam would enter the offline mode and let play games which can be run in offline mode, but apparently if the internet appears to be available but no connection to Steam servers can be established, then Steam freezes and doesn't know what to do.

Wow, how similar Galaxy appears to Steam in that regard. EDIT: Maybe like what JMich suggested.

var oXHR = new XMLHttpRequest();
oXHR.open("GET", "http://www.mozilla.org/", true);
oXHR.onreadystatechange = function (oEvent) {
if (oXHR.readyState === 4) {
if (oXHR.status === 200) {
console.log("Proceeding online") ;
}
}else {
console.log("Going into offline mode");
}
};
oXHR.send(null);

Funny how I can solve the problem doing a 5 second internet search which Valve is seemingly struggling for decades.
I say seemingly because I know it's fucking intended.
Stay strong and believe in the "sunrise patch" everybody.

Indeed. Check a site to see if you have internet access, assume you do when you get a true response, try to visit a second site that the firewall blocks.
Unless of course you mean checking for that specific site before visiting that specific one.