Gajeel's iron is neither weak against fire nor ice. Natsu managed to melt Gajeel's iron because of the overwhelming heat he could produce. The flames from fire dragon slayer magic are hotter and more destructive than regular flames. Gray has froze people successfully on multiple occasions, but any reasonably powerful character has broken out of it eventually. Freezing is Gray's most basic attack, and it has never been used to defeat anyone of decent enough strength. I think Gray can freeze Gajeel briefly, but Gajeel should be able to break out it without much difficulty.
Speaking of weakness, Gray's ice is weak against fire in the most literal sense. This has been highlighted to the readers the few times Mashima had the opportunity to emphasize so. Base Gray and Gajeel should be relatively equal. At the peak of their respective powers, Gray is more powerful than Gajeel. Not necessarily because he's the better fighter, but Gray (and to a larger extent, Natsu) has stronger emotions/PoF that they can tap into.
1) Gray performed better in head to head match ups. The two times Gajeel fought Natsu, they concluded with definitive losses for Gajeel. In their most recent fight, Natsu wasn't fighting seriously at the beginning. He busted out LFDM but the last time that form was genuinely relevant was back in the GMG arc. Even then, Natsu still treated the fight as a friendly competition till the end. He wasn't emotional. He wasn't worked up. However, Gajeel were. Natsu was at most annoyed by the fact that Gajeel was too stubborn and prideful to concede defeat when he could no longer put up a fight. He was on the verge of KO until Levy had to bail him out by using her pregnancy to distract Natsu.
Natsu then had enough left in his tank to fight Wraith, chase down and defeat Wraith-possessed Titan Makarov, before confronting the white witch in the back city church. The fight with Gajeel barely took any wind out of him. If we are being honest, it felt like a warm-up bout for Natsu. Gajeel's DF is more powerful than his ISDM, and so is Natsu's END form more powerful than LFDM. When Natsu fought Gray, he was in a mindless rage due to believing Lucy was dead and his vehement pursuit of hunting down Zeref. So was Gray because he thought Juvia was dead and believing END was the culprit behind his misery. Their fight was locked in a stalemate, till Erza eventually interrupted. Shortly afterwards, they both passed out due to exhaustion. Objectively speaking, Gray's display was notably more impressive than Gajeel.
2) Gray has a proven track record with PoF. I think base Natsu, Gray and Gajeel are relatively equal. Gajeel being more stealth and defense oriented, Natsu with the strongest attack, and Gray the most balanced and well-rounded amongst the three. However, when the stakes get high, Gray and especially Natsu, have the tendency to pull out more from the deep end with their emotions. Gajeel's emotions are rarely sufficient enough for him to overcome his foe. He usually requires PoF to be accompanied by another random power-up for that to happen. Against Shadow Rogue, he was losing until he literally consumed the shadow to achieve a new dual element form. Gajeel was losing against Torafuzar and Bloodman, until he consumed the trace carbon and MBP and transformed into steel form and dragon force. Gray inherited Silver's devil slaying magic during Tartaros, and that was that.
3) Natsu's DF should be far more powerful than Gajeel's. Not all dragons are equally powerful. There is a gap in power between the dragons. And it should be quite a wide one between Igneel and the other parent dragons. Metallicana, and other parent dragons, held Igneel in high esteem. They were reduced to mere spectators in the fight between Igneel and Acno. If they were in the same realm of power, it doesn't make sense for them to not aid Igneel when we consider the impact the fight could have. Igneel was referred to as the Flame Dragon King, and stated by Zeref to possess the power that could highly possibly overcome his immortality.
Think about it. Zeref knew Irene was a dragon, but it was not shown he ever thought of Irene being capable of killing him in her dragon form. Irene owed Zeref a debt of gratitude, and all this while never seek Irene's help to end his life. Because she couldn't. Dragon Irene had shown more feats than the parent dragons. There is a gulf in class between the strength of Igneel and the other parent dragons, Metallicana included. Similarly, not all DF are equally powerful. Gajeel's DF would certainly pose a bigger threat to Natsu, who would likely require more than just LFDM to defeat him. But I don't see how DF Gajeel beat END Natsu or Gray with devil slayer markings and PoF, because they are a completely different world compared to Bloodman.
