Let me elaborate: Ultima VII was written for
, the Microsoft PC operating system that precedes Windows. There is no Windows version of Ultima VII, but DOS programs used to be able to run on old versions of Windows. They no longer can and now Windows users have to use emulators as well, the most common being [url=http://www.dosbox.com]DOSBox.
What does this mean? When you buy Ultima VII from GOG it comes for both Windows and OS X already set up with an emulator, you don't need to do any setup. Windows user get plain old DOSBox, but Mac users get the shiny great Boxer (which is based on DOSBox, but has a nicer interface):
http://boxerapp.com/blog/2012/10/21/positively-agog/
Don't worry that it's all emulated, today's computers are so fast, Ultima VII will run better than on the PCs of its time. The game will play just as it was meant to be.
However, the way it was meant to be played is not necessarily the best way to play it. There is a modern port called Exult, it replaces the programming of the original game with a new engine that run natively (i.e. no emulation) on modern system. It's open source, multiplatform and free. Of course you still need to own Ultima VII for the actual game content. I won't go into how to set it up, it's explained very well on their site.
http://exult.sourceforge.net (BTW, it's called OS X, or formerly Mac OS X, not Mac OS. Mac OS is the name of the old Mac operating system which has nothing in common with OS X except the name. OS X is based off NeXT Step, a Unix-based operating system)
