Hmm, this is actually an interesting match. Gildarts is definetly one of the strongest. However, Natsu takes this.
Gildarts is definetly higher than Natsu in base or even in FDKM. If Natsu would remain in base(Not the chapter 536 "base"), then Gildarts could pretty much stomp him. Even with FDKM, Gildarts would win medium difficulty.
However the changing factor is DF. While we don't know how powerful is he exactly in DF, we know that if he pulls out a FDK spell in this mode, he will be able to vaporize Gildarts. Completely. This will leave Natsu powerless at the end, but Gildarts will be defeated.
So in short, this battle will be really hard. Natsu will have to pull out his "ace" to win this.
Obviously, I won't use Natsu's "Power of the Guild". That power is insanely strong, but it's definetly not something he can normally do. This power most likely has something to do with Lucy rewriting Natsu's book. I hope Hiro explains it at long last, so that people can finally stop calling Natsu with this mode "base".
P.S
It's so funny. Last tournament, Natsu won despite being weaker than the others. However in this tournament, it's completely different. People will vote against Natsu just because of their hatred of the character.