This is effectively a pawn-down gambit line for Black. If you're going to play it, be psychologically prepared. White does not have to go for the Grunfeld if they don't want to. The key line is 4. Qa4+, and then:
4...c6 5. dxc6 is a pawn down gambit. 5...Nxc6 6. e3 - Black has a slight lead in development and White's queen is somewhat exposed, but White ...
Black can only transpose to the Grunfeld if you play Nc3, so there are other candidate moves you can try to prove Blacks play is unsound.
(The idea being that we want to play e4 against Nxd5 and Black can't play Nd5xc3 or if Qxd5 then Nc3 gains time).
4.f3 seems the most annoying as the thread of e4 requires immediate ...