Many games say they aren't (for example Arcanum totally glitched up on my 10 and of course wasn't optimized) but I honestly question, if many of the ones that lack the optimization label, aren't so much the fact they aren't optimized but rather Windows 10 is fairly new and GOG just hasn't gotten around to updating many of the titles?

The Operating Systems listed on a game card aren't what the game is 'optimized' for, but what it's been verified to work on.
Compatibility, not optimization.

Okay, let me rephrase my question, replacing optimization for compatibility.

My question is still the same, the ones that aren't labeled, will they all not work, or is it possible that GOG just hasn't gotten around to updating the compatibility lists?

I don't remember them updating anything for Win10.

Most of the games say "compatible with Windows 10" so yes they did update the compatibility lists after Windows 10 came out. (Or at least most of them)

Can someone just please give me a straight answer?

Talking about two different updates methinks.

He's talking about updating the game itself to make work with windows 10. You're talking about them saying the game works in windows 10, and updating the store page to reflect that.

Just because they've updated the store page to say "windows 10" doesn't mean they've updated the program itself.

And the ones that aren't 10 compatible, I imagine they're looking into ways to make it work.

As to your original question.
Its a game by game basis, and a YMMV situation.
I recall someone posting somewhere that they got a game not listed for win 10 working, but the page said it wouldn't. (No, I don't remember which game. It was something I don't own, so I didn't bother to remember.)

If you want a better answer you'll probably have to list which specific games you're interested in, and then maybe ask in the game forums themselves.

Thanks :) Though I guess I can just play the games myself and do trial and error to see if they work or not :) though personally, "certain" 3D games have extremely terrifying glitches which I'd rather avoid.

Basically, it is as molerat said, its on a game by game basis and a YMMV situation. I have played Dust: An Elysian Tail on Windows 10, even though the GOG game card for it says it is only compatible with 7, 8 (and possibly XP?). I heard it needed an updated redist in order to work on 10. Otherwise, some games will absolutely refuse to work on 10. I haven't met such a game yet. It is usually advised to follow the game card and see if the game supports Windows 10. If not, check the forums for the respective game. You'll possibly find a thread or two regarding compatibility on modern operating systems, which could help with such cases.

Yeah that's what you get when people don't ask clear questions.
I'm guessing most games will work as well on 10 as they do on 7, but they're not supported on 10.
Which can mean they never got around to testing it or even that they never bothered to change the game card.
Basically 'buy at your own risk'.

Both. Some they haven't gotten to work as well as they'd like, and others have yet to be checked. In both cases, I would assume that GOG are looking into it and try to find ways to get the games running on the latest version of the OS.

Those that claim to be compatible are guaranteed to be, and GOG will help to the best of their ability if you have trouble anyway (and may even refund the game if you can't get it to work even with their help).

Sorry I was rather vague in hindsight ^^; I personally never assumed the other, and thought my question was straightforward.
Yeah I can always ask on the respective forum on getting Arcanum to work, because that game looked really promising. Thanks :) as I noted above, I'll try that with any game I can't get working.

Though, personally I'm scared of trying Sam and Max: Beyond Time and Space, due to Save the World glitched out horribly during the ending credits of episode 5 despite I was compatible at the time.

All these NPCs were massively stretching, filling the entire screen and their fingers became horrendously long and gnarled. I have never seen a scarier glitch (firsthand) in my life so I am not looking forward to "testing" the later games.

Well… it's a Telltale game, so bugs are to be expected. Especially if you don't use the Steam version. I keep reading about the invisible character bug (I think it was Sybil) that hasn't been fixed (on GOG) although it has been found several years ago.

Too bad you haven't recorded it XD