I don't know about IPOD but maybe is the same way to solve this problem, let's you see about my situation:
today i just put many songs in my iphone4s and one of it is gray. i can fix it already, i hope you try my way and i hope you can fix it too.THE WAY IS you just select MUSIC-->ARTISTS and left-slide to delete the name of artist(s) which has(ve) problem in your iphone4s. Now You will see that artists' name are deleted, that means the gray list of song will delete too. Then you can sync the songs which you like again. and no proplem any more. I hope it's work for you.
This is normally caused by an incomplete sync/Wi-Fi sync. Try syncing your device to your computer again and see if you are able to play the songs then.