In my opinion, I think the "power up" that Gray received in this chapter will be useless against END. Simply because by hype and powerscaling, END should be the third strongest character in the manga (being above Igneel but below Zeref and Acnologia). END is stronger than the Celestial Spirit King or Motherglare, who both can casually destroy multiple mountains at once.
I just don't think that Gray's new power will be relevant simply because I don't see this new power being at that level of power. Heck, Silver is a lot weaker than Mard, and END is far superior than Mard. I just don't see Gray being relevant in defeating END.