No potatoes, no rhubarb leaves and no avocado apparently - I hadn't heard the latter until a thread on here a few weeks ago.
Mine likes lettuce, curly kale, carrots, apple in limited quantities, banana (tiny portions very occasionally because too soft for his teeth),fresh basil (did I say that he is a little spoilt?), melon with the rind on, dandelion but not broccoli, parsnip or cucumber! And, of course, he eats hay and Timothy grass by the bushelful!
IME, all guinea pigs like slightly different things so you need to experiment a little. Also, the previous GP we had had food fads (no other way to describe it) - he would eat broccoli, then go off broccoli, then after a few months back on the broccoli again, ditto with carrots....