Er, Valve have confirmed this is the last story that they'll use Gordon Freeman as the protagonist.
Unless they fail to follow through like Microsoft on Halo.
I'm not sure where you got that idea, Valve have never once said Half-Life 3's story would be Gordon's last, since that would just be a really easy way to cut their franchise short even if Laidlaw has more ideas for it.
They've said "Episode 3 is the last story in the Half-Life 2 Saga, but not the last in the series.". Considering Episode 3 isn't in development and hasn't been for a while - even that quote is kind of pointless.