1. Dragon Slayers like Natsu have been able to eat all sorts of things that have nothing to do with his own element and can still incorporate it into his body. Other Dragon Slayers, especially the First Gen, are portrayed as having similar physiology and feats. Assuming that it's a NFL basically translates to not being able to Adapt to anything at all, which isn't true. We don't KNOW if it's true, but it's happened before, so it's an idea we can entertain.
2. Nacht was still completely bodied and toyed with actual Supreme Devils such as Lilith and Naamah even before they fused. So no he's not as strong as Supreme Devils
3. He's not Supreme Devil level, but yeah I'm reading on his page that he does get stronger so that's fair
4. Lieb's resistances to Devil hax come from being exposed and attacked by it when he was a toddler. This should apply for other characters damaged by Devils as well.
5. if Gajeel sees Nacht use shadow magic he will immediately start using Iron Shadow Dragon Mode in order to attempt to abuse his shadow absorption. At this point, anything he does can 1 shot Nacht just by touching him or any of his attacks or even his aura, since both characters's magic power will permeate the air, Nacht via Mana Room and Gajeel via pretty much just powering up or using any of his AOE. Not to mention, Nacht doesn't start with Felis, which is bad since A. 1 hit is enough to corrode his entire body, and 2. Nacht doesn't just ultra instinct in character
After doing a lot more research and basically rereading Black Clover, it doesn't seem like Gajeel will have a good time if he gets madness manip'd unless h manages to anime protagonist the power into his body. As mentioned in #4, every character in Black Clover should have resistance to Devil hax due to taking hits from Devils. Since Gajeel resists noe of that, once Nacht uses Mana Room he imediately goes insane and there's not really anything he could do. Sure he could 1 shot with Shadow corrosion but he needs to activate that mode first, and that's not happening before Nacht uses Mana Room
switching my vote to Nacht