If you were shooting with conventional analog cameras then yes, the gray card would be confined to exposure settings only. But the more sophisticated digital cameras also use the gray card to set manual white balance. For white balance you can either use a pure white object (i.e. a clean sheet of bond typing paper), or better yet...a gray card - which is more accurate. When you preset, the camera is evaluating the light on the gray card and then determing the correct white balance setting so that colors are exact and you won't have to suffer from color distortions (i.e. the overly yellow or orange cast casts when you take photos inside). It is a simple procedure, that when done correctly, yields very accurate results.
You can buy gray cards anywhere, but 99% of them are made from cardboard with a painted surface. Problem is that they wear out quickly and eventually become inaccurate. I found out about a nice place - Calumet Photo that sells high quality 8 x 10 plastic gray cards that come with a lifetime warranty. I ordered a few from them and was surprised at their quality. Their web address direct to that card is: