Well, yes and no.
Since they are special made from.. umm... Yamaha and SEGA there is 3 areas. 2(normaly) Audio tracks, Data and the 3rd the game itself.
If you put your GD-Disc in your CD-drive there is sometimes goddies for your PC there.. such as wallpapers and so on.. like in Sonic there is some cute wallpaper on thoose "tamagotchi"-wannabies. :P ect
If you play the disc in a CD-player you will notice 2 audio tracks as I mention. One for saftey if you are dumb and think that it is a regular CD with music on. There will be some audio that tells you that it is a Dreamcast disc and should not be played in a regular CD-player and the other one the audio for the game
But... you cant play them on a emulator. Tried with Ready to Rumble2, Sonic 1 and 2, Phantasy Star Online, Soul Calibur and the other ones that I had I ignored. But if you have any backups they will work just perfectly.
You can read more about it here
http://en.wikipedia.org/wiki/GD-ROM