I'm relatively new to GoG and modern PC gaming. Its cool to go back and pick up all the titles I have missed throughout the years.

The problem is I'm not sure which games will work properly on Windows 8.1. So far Ive been able to find out the answer to a particular game in most of the games forums or Ive been lucky with my choices. But its been close. I almost bought Fallout 3 for Steam the other day and luckily I saw in the player reviews that it doesn't run on Windows 8. But there has got to be a better way...

Is there a site to go to and check if a game is compatible with Windows 8.1?

http://www.gog.com/games##sort=bestselling&system=windows_8&page=1

So if a game is not checked off under Windows 8 it wont work? Or is it that those are the only games GoG guarantees will work on Windows 8?

Also, does Steam have the same filter?

Unlisted games might also work (and quite often do), but GOG does not guarantee it, and by default the GOG.com money-back guarantee doesn't cover those unlisted games, if you have technical problems with them in Windows 8.x.

The reason might be that GOG hasn't tested the said game on Windows 8.x (yet), or they encountered some occasional problems on certain Windows 8.x systems (while on some other Windows 8.x test systems it ran fine). Or, then the game has generic problems on all Windows 8.x systems. No guarantees.

No idea about Steam. Valve (Steam's owner) does not do any compatibility testing of their own like GOG does for old games, so it is up to the game publisher in Steam to list compatible OSes and provide support for them.

Not necessarily. It just means that the game isn't officially supported on Windows 8, because it failed at least some of GOG's compatibility tests. There is a chance that you can get it to run, depending on your computer's setup in terms of hardware and drivers, but if it fails, the lack of official support would mean that GOG's team won't be able to help you and you're not eligible to get your money back. It's essentially a gamble.
However, it's also possible that a game may fail to work properly on your system, even if you meet all the system requirements, but then support will help you.

It's probably best to check a game's particular sub-forum prior to purchase for topics about compatibility issues on a not-supported system or to ask here, because someone is likely to have tried it.
Sometimes, there are inofficial work-arounds or patches to be found in the forums to overcome these issues (for example, someone apparently found a fix for the Windows 8 problems with "Shogo - Mobile Armor Division")

You can also check out this list for GOG games that aren't officially supported on certain systems. There are far more games that run on Win 8 than games that don't.

I'm running Win 8.1 myself and I've had little problems with games not running on it, except for a handful of exceptions (e.g. Soulbringer doesn't run at all, Sacred doesn't run as smoothly). But there are several games on that list that run flawlessly on my rig despite not being supported for Win 8.1 (e.g. Alan Wake, Kentucky Route Zero, Rogue Legacy, Project Eden, Bloodrayne 2, Rune etc.). Some game might have issues at first but can be fixed (Still Life). But if they aren't officially tagged as compatible with Win 8, it's always wise to check for compatibility issues online, like you already do.

One more thing. Take into account that there are lots of other moving parts too, than merely the Windows version.

Sometimes even merely updating your NVidia or AMD graphics drivers (regardless of the Windows version) might break compatibility with some old games (e.g. I recall being able to run Gorky 17 passably on this Windows 7 PC before, but when I recently tried it again, no go, it would even crash the whole PC. The only thing that has changed over time are newer drivers (and naturally Windows security updates etc.)).

Or having a CPU which is too fast for the game, or a graphics chipset which causes some issues with the game.

I don't know about how it's on steam, but on GOG it means that it's guaranteed that it will work. I think on Steam on the game page there is a notice: Notice: Fallout 3 is not optimized for Windows 7 and later. But i'm not aware of any filter. However you can get Fallout 3 running on Windows 7+, but it does involve a couple of hoops.

http://www.gog.com/forum/general/windows_compatability_games_that_do_not_officially_run_on_specific_versions_of_windows has some compatibility information.

Thanks for all the replies peeps. Learned a lot there. Ive been away from PC gaming for 15 or so years and got used to consoles. Got to readjust to getting games to work properly on PC, but I haven't had too many problems.

Plus, I have amassed almost 100 games between GoG and Steam for probably less than $70 with all the sales and bundles. I will never pay full console price again! Maybe for Zelda...cant get enough.