The Grunfeld is a really strategically unique opening, and what I mean by that is that it is hard to find the same ideas in other openings.
As others already mentioned, the idea behind this opening is that you are giving up the center but in exchange you get a very nice activity for your pieces, especially for the Bishop on g7 to start with:
Against the main line, you probably want to play c5, Nc6, Ng4 etc. These moves put a good pressure on White's pawn center (I know this is not a real position from the Grunfeld defense !):
To finish with the endgame, there is a saying "you should not play the Grunfeld if you are afraid of a white passed pawn on d5". Indeed, even if many Grunfeld endgames are playable, they can be slightly uncomfortable. On a more generic standpoint, it seems to me that many Grunfeld endgames at top level end up in theoretically drawn positions where White is one pawn up. thus many ways for White to press a little bit.
This is a video I have made to help intermediate players grasp the ideas of the Grunfeld defense: https://youtu.be/1kY-XJQYA3A