All this talk of possible worlds and modal logic serves to obscure the utter beauty and truth of your proof, Libertarian.
Thankfully, a simple observation will allow us to cut away all the machinery (and confusion) of possible worlds.
The proof shows, from the premises (G -> G) and <>G that we can conclude G. In fact, careful inspection of the proof shows that it contains a stronger conclusion, namely, G.
That means, from the single premise (G -> G), we can conclude <>G -> G. If God exists in one possible world, he exists in all possible worlds.
Obviously, the contrapositive is also true: ~G -> ~<>G. Or, shifting the negation inward on both sides: <>~G -> ~G. That is, if God does not exist in even one possible world, he does not exist in any possible world.
Thus, all possible worlds agree on the truth value of G. And since G is the only atomic proposition in the entire proof, we don’t need to speak of possible worlds at all; we can just choose a representative world, say this one.
Then all the modal quantifiers can be eliminated by noticing that P (P is necessary, where P is a proposition containing G as the only atomic proposition) will have the same truth value as P, and also that <>P is the same as P.
Thus, the first assumption, (G -> G) is really just G -> G, which is really just G -> G.
And the second assumption <>G is really just G.
Not only have we simplified the assumptions (to “if God exists then God exists” and “God exists”), but we’ve simplified the proof too because we don’t have to have a bunch of extra steps to juggle those meaningless boxes and diamonds:
- G -> G Assumption
- G Assumption
- G Modus ponens, 1 and 2
In fact, even the simplified proof contains a couple of extra steps.