Is it possible that Goku is more powerful than Beerus in 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..