This is hard. If Mashima wants this to end as Naruto vs Sasuke or Ichigo vs Uryuu, that Gray will need to win fight against Natsu or/and Gajeel.
How Gray sounds, it same as Sasuke, so if we are going by that Gray will win this fight if it comes to Natsu vs Gray, that wouldn't be too weird, consider Gray was little stronger than Natsu at the end of Tartarus arc, and considering that he is currently antagonist, that antagonist power up could even give him more advantage over Natsu. But problem is Gajeel, it's really hard to believe that Gray is currently stronger than Natsu and Gajeel combined.