In the end, the Ontological Argument fails as a proof for the existence of God when careful attention is paid to the cognitive terms that it employs. When the terms are disambiguated, either nothing philosophically interesting follows or nothing follows at all.

What is the ontological problem?

Many classical philosophical problems are problems in ontology: the question whether or not there is a god, or the problem of the existence of universals, etc.. These are all problems in ontology in the sense that they deal with whether or not a certain thing, or more broadly entity, exists.

What was Gaunilo's main problem with the ontological argument?

In his work In Behalf of the Fool, Gaunilo contends that St Anselm's ontological argument fails because logic of the same kind would force one to conclude many things exist which certainly do not. An empiricist, Gaunilo thought that the human intellect is only able to comprehend information provided by the senses.