How should I play against the Grunfeld defense as white? What is the best defense as white against the Grunfeld Defense?

I have been playing the Blackmar-Diemer Gambit over and over again, I suspect some opponents notice this, and open up with 1... Nf6, so this gets me off guard.  I usually try to continue the Blackmar-Diemer Gambit, but black normally goes a different route.
First off, 1...Nf6 against 1. d4 is not the Gruenfeld Defense: this opening is defined by an order of moves 3 deep. See the Wikipedia page.

In your case after 1. d4 Nf6 simply play 2. Nc3 and you will be well on your way to getting a Blackmar-Diemer, because if Black doesn't play 2...d5 you will play 3. e4 and you will get your pawn to e4 without having to gambit it: almost like an improved Blackmar-Diemer.

But if Black plays 2...d5 in response to 2. Nc3 then 3. e4 is the Blackmar-Diemer, after 3...dxe4 4. f3, just via a different move order (a "transposition").  See Wikipedia's page for the Blackmar-Diemer.