A plain pellet is far, far better than a mixed feed like Gerty any day of the week. There are certain bits in Gerty that are healthier than others (although quite a few things they really don't need in their diet at all) and if you have more than one pig, you don't know if one pig is getting all the junk (not that you really want any of them to be having it!). With a good quality plain pellet, you know what they're eating and it what ratios.
What I dislike most about Gerty is that they use artificial colourings in the food (those brightly coloured biscuity bits). Why on earth do they think it's necessary to introduce crud like that in to a guinea pig's diet? The CCT are very anti-Gerty for all the rubbish it contains, and quite convinced that trying to process these chemicals puts undue stress on their livers and kidneys too.
For guinea pigs over six months of age, a timothy based pellet is ideal. Oxbow's Cavy Cuisine seems to be the only one we can get in the UK
http://www.bunnybazaar.co.uk/refills.html. However if you can't get it or think the price is a tad high then I think Burgess Guinea Excel is the best you can get over here. It's so much better balanced for their nutritional needs than Gerty. Have a look at this page for the pros and cons of some commercial feeds
http://cyberpiggies.coolfreepages.com/brands.htm.