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Do you mean the Grunfeld defense? You only have to know the line your opponent plays. :-)

Yes, there is a new rule in chess that they have been debating for the past few years.  FIDE voted years ago that in order for a chess player to officially claim that he plays an opening, he must demonstrate complete knowledge of all repercussions of every move, including a 15-ply demonstration with flawless clarity.  The sides were very heated, but they finally came to a conclusion today that:

#1 If you don't know how to play, then don't play.

#2 If you ain't playing to have fun, then it doesn't matter.

#3 It is spelled Gruenfeld, not Gruensfield.

#4 When you learn all of the lines in this defense, give me a call and I will adopt you as my trainer.

It's pretty complex so it is not a bad idea to learn the major lines and maybe some of the other, less common lines (ie 1.d4, Nf6 2.c4, g6 3.Nc3, d5 4.cxd5, Nxd5 5.Bd2, which is a difficult line to play against and should be learned if you are either color)

Just read the chess book " Grunfeld Defense" , by Kytianin, Estrin,Moscow, 1959 year. 

Or Prins's book: " Grunfeld Defense at the Chess Practice", 1940 year.

Or Grunfeld " Pocket book for debuts", 1950.

M.Eive "Theory of Chess Openings",1953, 5 edition