the grunfeld became the main choice against d4 at gm level. remember the candidates in kazan, when the grunfeld did so well, except in the last gelfand-grischuk game. remember kramnik playing reti/english against grischuk just to avoid this opening.
I think it changes from one tournament to the next. Although since I have been following chess since about 2008-9 I think the most common defence against 1. d4 at 2700+ tournaments has been the Slav.
does that include semi slav
does that include semi slav
Yes, so I suppose technically that's two openings. 
How deeply do you need to memorize the Grunfeld main lines as Black? I heard the Grunfeld can be up to 30 moves deep in theory for some lines. True?
Yes. This is also true for a lot of other openings.. The Semi slav botvinnik can probably go 50 moves deep into theory.. Same with the Najdorf, and the King's Indian.... That doesn't mean you have to memorize them all though, by the way, well except for the Semi-slav botvinnik.. and the sharp Bg5 line of the Sicilian Najdorf
This is what is slowly killing chess, imho.
Seriously, imagine watching a game between two GMs live and the commentators mentioning they're still playing book moves for 50 moves... wtf ?
the grunfeld became the main choice against d4 at gm level. remember the candidates in kazan, when the grunfeld did so well, except in the last gelfand-grischuk game. remember kramnik playing reti/english against grischuk just to avoid this opening.