Natsu need to confort Zeref as Natsu, being aware of his ancestry, after he is full capable of controling his Demon Powers, not just as END some evil demon. Otherwise there really wouldn't be point in Natsu being main character, good guy etc.
So I am thinking taht Gray at some point before Natsu vs Zeref, will fight him, well not really Natsu but END. END is hell out of strong, considering not even Igneel could kill him. Gray is human, but I am thinking that Gray will become demon in future (like when Dragon Slayers become Dragons), that will much strenght his powers. Actually, I am thinking that Gray is already in the proces and currently is at the stage 1. Next stage will be his Ethorious form. Thinking about that Gray's case is pretty similar to Sasuke's. When Sasuke got his Cursed Seal he was unstable (for the same reason Gray left Juvia and went to Porylisca). Sasuke died in that box and was able to master his Cursed Power. Gray didn't really die but I remember that parts that are black are considered dead (not sure how valid this is, I think it was from Wikia). So I really expect Gray's Demon form to be something similar to Sasuke's 2nd stage Cursed Seal.
Now, to return on the topic. With new power up, Gray will be a lot stronger. I don't think that will be enough to beat END, but END is demon and Gray is Devil Slayer, which give him advantage. I am thinking it will be enough for him not to kill END but to defeat him and return him to Natsu.
Also, I see Natsu and END as Naruto and Kurama or Ichigo and Hichigo. After being beaten by Gray, Natsu will be able to master END's powers and be conscious while using it (much like Naruto-Kurama, Ichigo-Hichigo). Than Natsu will fight Zeref.
So, yeah I am pretty possitive Natsu and Gray will fight, but I am also positive that Gray will not try to kill Natsu, but to save him.