Is it possible that Goku is more powerful than Beerus in Super Saiyan God Super Saiyan form? In DBZ: Battle of Gods, Goku became Super Saiyan God and he makes Beerus use upto 70% of his energy. So, is it possible for Goku to defeat Beerus in his Super Saiyan God Super Saiyan form?
There is no clear answer.
But according to the Wikia article on SSGSS:


  The form exceedingly surpasses its predecessor, Super Saiyan God, and
  all of the other Saiyan forms.


This source also mentions that SSGSS will appear in Dragon Ball Super, so there might be a chance, but we can't be sure unless we watch Dragon Ball Super..