Since AMD has let slip that Windows 10 will be out in July, the question now is, will all the games run flawlessly?

There is no way to know with certainty, but each game's store page lists the operating systems that the particular game is supported on. The games may actually install on work on other operating system versions but the ones listed are the ones that GOG and/or the game publisher has directly tested and found the game to work well enough in their test environment to consider offering official support for the game on that platform. Over time as new versions of Windows are officially released GOG tests the games in the catalogue on the new system and updates the store pages to list the new OS if they're able to get the games to work reliably with it and the publisher agrees as well.

I believe the entire catalogue supports Windows 7 now, and a large portion of it is supported on Windows 8.x but I don't think all games are supported on 8.x yet. Most likely this will be similar with Windows 10 and every few days/weeks they'll add Windows 10 as a listed supported OS for games they're able to test it with and get it to work to their satisfaction. It's unlikely the entire catalogue will work though because many games don't work on Windows 8 either and each new OS release drops some backward compatibility etc.

They do also however sometimes look into trying to find fixes/hacks etc. to get some of the older games to work on newer systems too though, so even if a game is not listed as supported on Windows 10 at first, it may show up at a later date.

Hope this helps.

There are [url=http://www.an-ovel.com/cgi-bin/magog.cgi?ver=500&scp=gdspu&dsp=ipgfsorlcmbaxyzXhDFGH051234&ord=&flt=onn~w07~&opt=&myf=TueMay190645202015_empty_0Z6tFVCNeqpMM]18 games[/url] that still don't support Windows 7. Oddly enough, some of them support Windows 8.

As an addition to the excellent answer above me, will mostly depend on whether DOSBox and ScummVM run on Win10 for most games. If DOSBox works, most DOS games will probably run without problems on the next windows iteration as well, and same goes for scummVM. The problem children are probably going to be the old native Win95/98 games. I'm looking at you, Interstate 76 >.>

Oh, sheesh. I just took for granted that all work on Windows 7, and Blood 2 is something I plan to play right after I complete One Unit Whole Blood which I'm doing now. I hope there is a patch/workaround or something, I'd hate to bring out my Windows XP machine.

It does play on my Windows 7 64 bit machine. In fact, quite a good number of those games could play on Windows 7. It's just that GOG doesn't guiarantee it.

Yes, I did a quick google and it seems results vary. I'll just have to see how it goes.

Quick answer:

As of now: No.
For the future: Most likely not.

GOG will surely announce official game support for Windows 10 as they have done before when Win 7, 8 and 8.1 came around. This will most likely happen a good while after Windows 10 has been released. Still, there will be games which simply won't be working on Win 10.

Many of those games on that list I've played fine on Windows 7/64bit. However, that is not a guarantee they will work fine on all Windows 7 systems.

And frankly, quite often the problems are more due to hardware (GPU) and drivers, than the Windows version. Take for example Gorky 17 (which is also on that list): on one of my Windows 7 machines (a HP laptop with Intel HD graphics) it runs just fine IIRC.

On another Windows 7 laptop (Geforce GTX 670M graphics) it currently crashes hard when the game starts (at least sometimes jamming the whole Windows 7 => hard reboot needed) in 3D accelerated mode. In software renderer mode it otherwise runs fine, but the audio has awful stuttering.

Also, Interstate'76 has problems even in Windows XP. The problems are more due to CPU speed than OS, I think. The game physics become broken on new modern PCs.

E.g. Gothic, The Longest Journey and MDK 2 from that list have ran fine on Windows 7 for me, as far as I remember. Most of the others I don't remember or haven't tried properly.

Most will, maybe not all. Very old games running on emulators (DosBox, ScummVM) will run for sure as well as newer games, but there might be a couple of games from the early Windows days which might not run that well. GOG will surely try all they can.

Don't worry.
Wine for windows is coming.
After that all your non-supported games will run without hassle.

Just for general information I solved the audio stuttering in software renderer mode (in Windows 7 64-bit) as follows:
1.) Right-click on 'Gorky 17' desktop icon and then select 'Properties'.
2.) Click on 'Compatibility' tab and change compatibility mode from 'Windows 95' to 'Windows XP (Service Pack 3)'.
3.) Ensure that 'Run this program as administrator' is selected and then click 'OK' button to close the panel.

If anyone is struggling to get this game running, there is more information in this thread:

http://www.gog.com/forum/gorky_17/some_solutions_for_gorky_17

I might try that, albeit I would be surprised if I hadn't tried the different compatibility settings already. The Gorky 17 audio stuttering in software renderer mode is quite a common problem, after all. It may depend on the PC/configuration whether that works.

If the 3D accelerated version runs ok, that usually doesn't seem to have the audio stuttering. For some reason it seems to happen with the software renderer only.

In Windows XP, the audio stuttering can be fixed by going to Control Panel Audio options, and disabling audio HW acceleration. Windows 7 doesn't seem to have a similar option. EDIT: Yep, you mentioned that already in your linked thread.

Anyway, let's see if your suggestion for the stuttering sound works. I think the game is otherwise fine in software renderer mode, it doesn't benefit that much from 3D acceleration as most of the graphics are bitmapped 2D.

Wow, I own 8 of those and I have played some of them in Windows 7 so they at least installed and started up and were playable at some point for me. In particular the UFO games all work in Windows 7 for me, and I played Gothic for a few hours one night to try it out. It's possible the games work but are unstable or buggy at some point perhaps. I'd have to go through them to find out I imagine. :)

It's also possible it's just a bug in the store page's listed supported OS also as that seems more common than not these days. :)
Good to see the list though!

I also confirm that The Longest Journey ran fine on my Window 7 64.