While reading the forums I noticed that there are several threads about playing game X from GOG on a Mac. Since I own a Mac and I also bought a few games from GOG I thought it would be a good idea to create a topic to share some possibilities with other Mac users how to play these games on their machine and to create a list that shows how every game performs using different approaches.
Great. First topic here and already a typo :/. Could a moderator correct this please? Thank you!
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How can I play windows games on my Mac?
1. Game-specific native ports/launchers/addons
The older and the more popular the game, the better the chances that somebody created a port, launcher, addon, etc. to play the game on other platforms (e.g. Freespace 2 or Duke Nukem 3D). In most cases this is the best method to run a game on OSX. Use google to search for "GAMENAME mac" or take a look in the GOG forums.
[+] Best performance (in most cases)
[+] Free
[-] Available for only a few games
2. Boot Camp
If you own an Intel Mac and Leopard you have the option to natively install Windows on your machine. It'll cost you some time, HDD space and probably money (if you don't already have a Windows license) but it will give you the most performance since it's basically like running the game on a PC. The biggest disadvantage (in my opinion) is that you have to shut down OSX and reboot in Windows in order to play a game. But if all else fails, this is the way to go. More information about Boot Camp on apple.com.
[+] Great performance
[+] Works with every game
[-] Time-consuming set up
[-] Windows license required
[-] You need to reboot your system before being able to play
3. Virtual Machines
A Virtual Machine is basically an application in which you can run Windows ("Windows in a window"). The VM shares the system resources of the host (OSX) with the guest system (Windows) which is also the biggest drawback: based on your system specs you may experience weak performance either on the host, the guest or both. However most of the games here on GOG are not that demanding when it comes to hardware so the performance you get from a VM will be enough in most cases.
[+] Sufficient performance (at least for GOG games)
[-] Most VM are not for free (and not cheap)
[-] Windows license required
Here are some options if you want to use a Virtual Machine on your Mac:
- VirtualBox (free)
- CrossOver Games ($39.95, 7-days trial available)
- VMware Fusion ($79.99, 30-days trial available)
- Parallels Desktop ($79.99, 30-days trial available)
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Game List
Beneath A Steel Sky
- Native: Gold (using ScummVM for Mac)
- VM (COG): Gold
Duke Nukem 3D: Atomic Edition
- VM (COG): Bronce (unplayable with mouse aim)
- VM (VMW): Silver (mouse aim is kind of choppy)
Duke Nukem Manhatten Project
- VM (COG): Gold
Freespace 2
- Native: Gold (using FS2_Open)
Haegemonia Gold Edition
- VM (COG): Unplayable (crashes immediately)
- VM (VMW): Bronce (very low FPS, close to unplayable)
Legend:
Gold = Great performance, no issues
Silver = Good performance, no game-breaking issues
Bronce = Bad performance, serious issues
VM = Virtual Machine
VBX = VirtualBox
COG = CrossOver Games
VMW = VMware Fusion
PAD = Paralles Desktop
Note: Boot Camp will only be mentioned if there are problems since every game should work using Boot Camp.
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Please feel free to post your opinions and experiences with Windows games on Mac in general or with a specific game (from GOG). I will try to keep up and update the list. I hope together we can create a solid list with every game available on GOG.

1. a similar topic exists (except not with this much info, at least in a single post): http://www.gog.com/en/forum/general/gog_games_on_mac.
2. Also, all games that run in DOSbox or ScummVM can be played through the Mac OS X native version of the respective application. The game will still have to be installed through (Dar)Wine or Crossover however, else the files cannot be extracted.

In the specific case of Beneath A Steel Sky, you can just download it from ScummVM.org instead. It comes in a ZIP file there, IIRC.