If the concept G is not instantiated by some real thing, then we can construct a 'greater' concept, G+, which is G 'plus' instantiation.
The standard objection to this inference is that there is no solid criteria that permits us to call the consequent concept, G+, 'greater' than G. My objection, by contrast, is that even if some kind of standard of 'greatness' is granted, G+ can only be formed if G is of a certain kind of concepts, and there is no non-question-begging way of guaranteeing the G in question is of the right kind.
For example, consider the concepts 'virtue' and 'cat'. We can take 'cat' to be of the right kind: 'existant cat' (or 'is a substance that is a cat') is, pace Kant, a perfectly legitimate concept. However, 'existant virtue' (or 'is a substance that is virtue') is illegitimate: virtue, in and of itself, is not some thing or some stuff! While we can say 'that thing is a cat', it would be absurd to say 'that thing is virtue'.
Hence, Anselm's argument plays on an equivocation, and an admittedly briefly survey of the literature suggests my recognition of this equivocation in the context of Anselm's argument is novel; however, as this doctrine is the central logical thesis of Aristotle's Categories, it can be used to target Anselm directly without danger of anachronism.