I was wondering if the Mac version of FS2 is available for download here? I bought the game, but can only find the .exe.

You have to get the Freespace Open stuff installed, and then you have to run the installers to get the data files out of it. Do you have a VM product or some version of Wine to be able to do this?
It's not super simple to do, but you end up with a very nice end product. It works beautifully on my Mac.

Note the system requirements on all the games available from GOG, most specifically FS2's:
"Minimum system requirements: Windows XP or Windows Vista, 1 GHz Processor (1.4 GHz recommended), 256MB RAM (512 recommended), 3D graphics card compatible with DirectX 7 (compatible with DirectX 9 recommended), monitor capable of displaying 1024x768 resolution, Mouse, Keyboard."
And the GOG "About Us' Page:
"4. All games are Vista and XP compatible.
Thanks to our handsome programming team, the classics are now Windows Vista and Windows XP compatible. Now you can use your lightning-fast PC to unleash the full potential of those games you just couldn’t play properly on that busted old 386."
In other words, GOG does not sell Mac games, just Windows games.

Yeah yeah, very nice. Not helpful though. It does work on Mac, you just have to unpack it somehow, and use the mac binary from FSO.

That's great, except he was asking if the Mac version of Freespace 2 was available for download here, not how to run FSO on Mac. I was just answering the actual question asked.

But yeah. GOG doesn't provide Mac(Linux downloads. Can't blame them for the popularity of Windowses in gaming usage.
The only option I'd dare to recommend is to install FS2 on a Windows computer, then move it to the Mac and install FreeSpace 2 Open. Not the easiest and most hassle-free option, but it'll get the job done.

Ok, granted. From my perspective you were taking the opportunity to give him a jolly good lecture instead of helping him get running on a Mac with the GOG files. I probably overreacted, sorry.

Frankly, I was being a little "lecture-y". The website plainly states in multiple places that the games are for Windows only. Given that, it seems silly to be asking if the Mac version is available here and even sillier to have bought the game in the first place without knowing that. To me, this is one of those "you should have known better" moments.

Still with us azile0?

FSO is a completely different engine, not just libraries. No, GOG has nothing to do with FSO, they have only distributed the original retail binaries. If you have a Windows machine, you can run just the GOG version without FSO (though you'll be missing a whole lot of fan-made updates).
You need FSO to run on Mac or Linux at all, because GOG doesn't distribute non-Windows-based binaries.
1- First, install the FS2 from GOG, somewhere. You may need WINE or an emulator or something.
2- Copy the whole folder over to your Mac. I put mine in /Applications/Games/FreeSpace2. You want the existing directory structure to remain the same. If you also have FS1, put the .vp files in the fsport folder
3- Next, download the FSO installer from here. When you run it, it will point to a DOS-based drive letter and path combo (C:\Games\FreeSpace2). Change this to the folder you selected in step 2
Choose one of the 4 types of installs, and let 'er rip. It will probably take quite a while.
Ideally what should have happened is all of the files were updated or replaced in the proper locations. You should have a data folder with the FS2 game data in there, as well as a whole bunch of mods on the same level as the data folder.
4 - Run the FS2_Launcher that was installed in the game folder. It might do its own updating to make sure it's the latest version. Try it plain vanilla if you can. Then go to the "GamePlay+Speed" tab and make sure the "Selected Mod" is "mediavps". If it wasn't, you should notice some pretty impressive upgrades. Drop down this list and see all of the other mods installed - like "fsport" if you installed FS1 data (this is a CONSIDERABLE upgrade).
5 - Beware the Shivans.
Lots more info here

FreeSpace 2 is a Windows game. It won't run natively on Macs. You could run it with Wine/whatever it is on Mac, but that is a bit of a hassle. FreeSpace Open is a modified, open source version of the FS2 game engine. There are different versions for Windows, Linux and Mac. FSO will run on Macs without Wine/whatever, henceforth it is conciderably better. If you visit http://www.hard-light.net/forums/ , you'll find a set of installation instructions for Mac at http://www.hard-light.net/forums/index.php?topic=60842.0 . They're at the bottom of the thread. Running FSO on Mac is easier (in the long run) than running FS2, plus FSO makes it possible for you to enable sweet graphical goodies and run an arseload of user-made campaigns and mods.
Well, basically what BoxOfSnoo said.