Definitely not grass and dandelions as their main food, although they're ok as a treat if they haven't been treated with pesticides or fertilizer and if other animals haven't urinated on it.
Guinea pig food has vitamins and minerals that the pigs need. That said, Kaytee is not a very good brand -- your pigs are far better off with Oxbow.
I don't see a Kaytee gourmet formulation of guinea pig food. Are you in the U.S.?
Pigs under six months need a food that is primarily alfalfa hay, pigs over six months need primarily timothy. Good GP food will have a calcium source that is NOT limestone (calcium carbonate). It should also have stabilized vitamin C added. And you should also be giving veggies with vitamin C -- bell pepper is an excellent source.