GOG would like you to know that they are “as ready as can be” for Windows 10:

GOG.com has always been about making sure our releases, especially the classics, are tested and playable out of the box on modern computers.

When GOG.com launched in 2008, Windows XP was by far the most popular operating system among gamers. Ever since then we’ve meticulously tested our entire library to introduce Day 1 compatibility with every new Windows release. We’ve done it for Windows 7, we’ve done it for Windows 8 – now we’re doing it all again for Windows 10.

So far, things are looking great. We’re encountering very few problems and most of those are either resolvable on our end, or likely to get fixed as Windows 10 nears release. In fact, many games that had issues on operating systems newer than Windows XP once again won’t suffer any hiccups on Windows 10. The OS is still a work in progress, so while anything can happen until the release date we’re confident for the future. This is the smoothest transition to a brand new system that we’ve ever worked on.

Some games may need a patch to run perfectly smoothly, but don’t worry – they will be available on the site, and if you use GOG Galaxy, your games will be ready to go automatically.

I’m assuming that the DOSBox-wrapped games — which includes basically every Ultima title (except Ultima 9) — will probably continue to work without issue, although I also expect the DOSBox team will release a Windows 10 update to their Windows build at some point, just for good measure. Ultima 9 actually plays really well under Windows 8.1, and I would assume that it will continue to do so under Windows 10.