There should be 6 CD's.
I don't know what the contents of the CD's would look like on a modern Mac.
However the CD's can't be read on a PC (at least not without special software).
It looks like there were separate Mac and DOS versions though, not a hybrid disc as I first thought. So if you're trying read the CD on a PC, you'd need a program like one of the programs mentioned here, which allows the PC to understand the file system used on old Mac CD's
http://www.computerhope.com/issues/ch000676.htmAre you looking at the CD on a PC or on your Mac?
I don't have a modern Mac handy to test, but I thought modern Macs could read the old Mac file systems.
Apple is not great for backwards compatibility though, and it's possible you'd need a special program in order to read Mac CD's made for OS9 and earlier. If your Mac came with reinstall CD's (or DVD's), the program to read old Mac CD's may be on the CD's (just not installed by default).
As for the CD being in a sleeve instead of the CD case, I notice I have an empty CD sleeve in the game box of my Mac version of GK2, so it's possible it originally came with one of the CD's in the sleeve. Not sure why they'd do that, but it's not unheard of.