Full Idea
For Frege (unlike Kant) existence is a genuine concept, but of the second level, not the first. Since God's perfections are of the first level, existence is not a candidate to be one of them.
Gist of Idea
Existence is not a first-level concept (of God), but a second-level property of concepts
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §053) by Michael Potter - The Rise of Analytic Philosophy 1879-1930 09 'App'
Book Reference
Potter,Michael: 'The Rise of Anaytic Philosophy 1879-1930' [Routledge 2020], p.60
A Reaction
That is, God's perfections are of God, but existence is a concept of concepts (that they are instantiated). So existence is a metaconcept. I'm not convinced. If I bake a successful cake, its existence is its most wonderful feature.
Related Idea
Idea 8644 Because existence is a property of concepts the ontological argument for God fails [Frege]