What Geach calls 'Frege's point' is the claim that the same thought or proposition may occur now asserted, now unasserted. Frege expresses this very clearly in a short essay*, posthumously published, probably written in 1915, where he argues that asserting a sentence is a matter of the assertoric force with which the sentence is uttered, and that assertion is not the function of the word 'true'.
His argument is as follows. Uttering a sentence 'sea water is salt' merely expresses a thought. Nothing is meant to be asserted (behauptet werden solle). This becomes clear when turn the sentence into a that-clause: 'that sea-water is salty'. The that-clause does not assert anything. Or we could have the sentence spoken by an actor on a stage, where the actor does not speak with 'assertoric force' (or at least only seems to).
We can make this even clearer, he says, by adding the words 'it is true' to the expression 'that seawater is salty'. This forms a sentence that we can also turn into a that-clause: 'that it is true that sea-water is salty'. Thus the sense of the word 'true' does not make any essential contribution to the thought. If I assert 'it is true that sea-water is salty', I assert the same thing as if I assert 'sea-water is salty'. Thus the assertion is not to be found in the word 'true', but in the assertoric force with which the sentence is uttered.
Readers of this blog will recognise this as the thesis that William Vallicella has been defending from his website in Arizona. I shall discuss Frege's argument in my next post.
* "My Basic Logical Insights" (Posthumous Writings, transl. P. Long and R. White, 251-2. This is a translation of Nachgelassene Schriften 271-2).