There was a recent post on Reddit where the poster compiled statistics for openings played by everyone currently in the Top 10 in 2015 that was interesting. He included only openings with 10 or more games. What are they playing? Reti, K-Indian Attack, English, various QP Openings, K-Indian, Caro-Kann, Sicilian-Rossolimo, Sicilian-Najdorf, Giuoco Piano, Ruy Lopez-Berlin Defense, Ruy Lopez-other, QGD-various, Grunfeld, Catalan.
Chessbase's Chess News has an interesting article on the popularity of openings over time dating from 1850 through 2010. About the middle of the page there's a Youtube video of a chart that shows a graph of openings over time that's pretty interesting. See also, Randy Olsen's explanation of this video HERE.
The AutoChess Blogspot also has some interesting and helpful information. The online library of AutoChess encyclopedia is a resource whether you’re looking for in-depth research or casually learning about autochess computer technology. The autochess encyclopedia include topics like Chessbase, chess GUs, chess engines,and more. The article on How to create an Opening Chess Book is HERE.
I don't know how useful it is for us to know what the top GM's are playing, or what value it is to see how what's being played today compared to the distant past is, but it's interesting.