I feel it depends on the mix I would guess? I was in a shop at the weekend that sold animal bits (not a pet shop) and was looking at the ingredients of some, curiousity really, and found that some where really unsuitable as it contains seeds!
I have never had any problems with Gerty. My guinea pigs that live with my mum always are changing which food they like all the time. They eat everything but then after a while they decide they want a change
My piggies are on Spillers Guinea Pig Muesli, and they love it. I make sure not to overfill bowls so that they eat all that is in them, rather than picking out what they want Sometimes I buy a bag of the PaH nuggest and mix them in, so that they have some variety. They really enjoy them ;D But like all piggies, mine love their variety of vegetables. As long as they are getting those they are happiest, lol.