Problem in grunfeld

What will happen if someone plays an Anti-Grunfeld against a grunfeld player 

Same thing that happens if you eat pasta and then anti-pasta.

If by the "Anti-Grunfeld", you mean the line 1.d4 Nf6 2.c4 g6 3.f3, usually dubbed the "Anti-Grunfeld", Black has 2 choices:

1) Play 3...d5 anyway.  You are simply in what is called the "Anti-Grunfeld".  Supposedly, this is a bit stronger for White than main like Exchange lines.

2) Play ...d6 instead of ...d5, whichever move you do it at, and in essence, you are now in a Saemisch King's Indian.  More often than not, White will directly transpose into the Saemisch KID.

Those are the 2 main choices. 3...c5 is another path, but if you wanted to play a Benoni/Benko you probably would have gone 2...c5.

Then there's this:

If black transposes into a Saemisch KID then won't he be worse as he will know less theory than white 

And there's another anti grunfeld