I don't think Andrew quite understands the situation. Giga files (.gig) are bundles which contain both audio files (samples) as well as the Giga instruments (which are the equivalent of EXS instruments). So converting them to wav simply wouldn't work.
The reason you're not seeing the instruments that come with Logic at the above-mentioned path is because they're located in yet another sampler instruments folder -- the one located in the main (root) sampler instruments folder. I'll explain...
As you probably know, OSX is designed a multi-user operating system. This means that there's a main (root) folder system that contains all the usual stuff like applications, a library/application support folder of its own, etc. Then, each person ("user") working on that computer has an account. You probably have an admin account so you're the "big cheese" main user of the system with admin privileges. Evidence of this is your Home folder which contains a similar folder structure to the "main" folder structure and includes a library/application support folder of its own. Any files placed in the "main" library/application support folder are available to all users on the system. Files placed in the users/me/library/application support folder are available only to that user. I suggested putting your files in there to keep them separate from the "main" sampler instruments folder, just to keep things clean as you're working on this install.
So, in summary, you're going to see Logic's stock, factory sampler instruments in:
HD/Library/Application Support/etc... sampler instruments folder.
And indeed, you could try putting your .gig files in that folder. But I don't think it's going to make a difference, as you're not able to see the .gig files in any other hue than grey, meaning that they're inaccessible. Funny that you can drag them onto your HD but that they turn grey. Not sure why that would be. If the DVD is Mac compatible (or native) then I don't see why you couldn't copy the files over. (Maybe Rounik, who's quite the Mac expert, could chime in on that. I'll ping him.)
But short of seeing any success copying the files over, there's a program called CDXTRACT (http://cdxtract.com/) that will convert .gig files to EXS-24.