Actually, both Discodan and Darksociologist are wrong. If we accept Discodans two premises, G indeed follows, but as Darksociologist points out, his reasoning is wrong.
~G --> ~(P --> A)
is equivalent to
(P->A) -> G
in classical logic (which I don't believe in, BTW). From ~P it follows that P->A. Combining this with (P->A) -> G, we have G.
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