So Gog's games like freespace two - often list windows xp or vista as the operating system required. Do the games work on Windows ten? Interested in picking up freespace 2 and maybe 1 (assuming they work well with keyboard mouse controls) but don't want to pick them up with they are not going to run on my machine...

Thanks!

I don't know about freespace specifically, but if you're motivated enough you can get (almost?) every game to work in Windows 10 - even if it means running a virtual machine.
If you're not willing to put in any effort, then just go with those requirements and don't buy the game. Again, it depends on the game; sometimes that "effort" is really minimal.

The OS version mentioned in the requirements near the bottom of the page aren't comprehensive; they sometimes just list the oldest version or two of Windows that the game can be used on.
The "Works on" entry on the right side of the page nearer the top, just beneath the screenshots, lists all the Windows versions that GOG will support the game on. (If Win 10 is mentioned in the sys reqs, but not in "Works on", it's probably an oversight.)
If Windows 10 is mentioned in neither place, it's "buyer beware". It may work fine out of the box -- or you may be able to get it working with a little messing around -- but GOG's testers apparently weren't able to get it working reliably.

I would check the game specific forums. There is a chance that for most users the game would work but GOG can't certify it because there are some cases when it doesn't. You just need to find out if those apply to you.

Accoding to the Freespace 2 game page - Works on: Windows (XP, Vista, 7, 8, 10)

As said, if GoG doesn't list it in "Works on", they aren't guaranteeing it. It means that they have had problems making it work reliably on at least one of their internal configurations. Oftentimes certain hardware causes problems. It could be problems with one manufacturer's graphic cards (AMD/NVIDIA) or some subset of their cards.

What's important is that GoG will not guarantee the game will work and as such are not required to give a refund if it doesn't work.

Most games will work on Win 10 anyway, but the risk would be on you.

Edit: Well, Freespace 2 is listed by GoG as working on Win 10. The above applies to the small number of games here which don't list Win 10 under "Works on"