Gaunilo was one of the earliest critics of Anselm’s ontological argument. Gaunilo doesn’t really specify any fault with the argument itself but argued that the argument is incorrect due to it’s genericity - i.e “God” can be replaced by almost anything.
When I heard about Anselm’s Argument for the first time, I must admit - I immediately had a revelation about the “Greatest Cookie Possible” - i.e the “Cookie than which no greater can exist”. I experienced an epiphany that this cookie would be even greater if it existed in the real world and not in just my imagination. In fact, it would be much better if it was right in front of me and I could eat it. As I reached to take a bite, I awoke from the false reality I had created and rather than a giant, heavenly cookie, I was greeted with curious expressions from my classmates.
This must have been how Gaunilo felt when he envisioned a perfect island. Gaunilo says that according to Anselm’s argument, the island (or, if you prefer, giant cookie) must exist. Since it does not actually exist, this must mean that Anselm’s argument is incorrect.
Well clearly this is one of the weaker negatives of the argument since - especially in my argument for the existence of the “Cookie than which no greater can exist” - the definition of greatness is subjective. I stated, for example, that “it would be much better if it was right in front of me and I could eat it”. This is better for me but definitely not better for the cookie.