The thing is that against Gildartsz Natsu had NO CHANCE at all, I mean Gildartz survived a solo battle against Acknowliga, an enemy so powerful that even the strongest members in the guild (including the current master) couldn't even touch.

On the other hand Rufus and Gray were sufficiently evenly matched that the power of emotion gave the boost needed. Even Mavis acknowledges Gray had a chance against him.

In short, Emotion in fairy tail makes something possible a reality; but it won't make something impossible possible.