My guinea pigs have a bowl of Wagg guinea pig crunch (which in my opinion is a good muesli) and a bowl of nuggets ranging from burgess excel (plain, mint and oregano, cranberry and blackcurrent), pets at home nuggets, science selective nuggets and I have just brought them a bag of Harringtons nuggets to try as they seem to get board with one particular brand of nuggets so I try and vary it.
I am not a fan of gerty myself due to the bright colourings and additives but everyone is different.