FDK Purgatory was one of the reasons Skullion decided to retreat. In round two of their fights with Diabolos, both Erza and Natsu manage to turn the tables on Kyria and Madmorl respectively. Just as Gray gets back to the ship and is set to join in, Skullion decides it's time to cut and run before fighting Gray again. He knew he was outmatched with his teammates weakened and Wendy neutralizing his offensive magic.
Those factors contributed to Skullion making a calculated decision to retreat as the odds were against his favour.
If he does resort to them, it would depend on which FDK spell Natsu employs to defeat Gajeel. There is an evident difference in the strength of the FDK techniques in Natsu's arsenal. FDK Demolition Fist failed to cause Madmorl to flinch, while Purgatory could at least put him down for a few seconds, only for Madmorl to quickly get back to his feet again with superficial burns.
I guess, technically, we can count that as a loss for Madmorl, similar to how Gray, Erza and Natsu lost to Skullion, Kyria and Madmorl respectively in their first fights. However, if Gray did fight Skullion a second time, he could very well turn the tables like how both Erza and Natsu did. Therefore, if Gajeel were to fall to an attack short of being as powerful as FDK Purgatory, but rather the level of a Demolition Fist, I would hesistate to put him Gray's level. If Mashima decides to let Natsu bust out a new FDK tech which he has not shown before, which is another possibility, I would be even more unsure.
Of course. Gray's insta-freezing feats are lacklustre, and I have rarely seen him pull that off on any decently strong character. He most commonly succeeds insta-freezing fodders like the grunts and foot soldiers on the enemies' side, but the strongest named character that I can recall him freezing were Tempester. However, that was an Etherious with a devil slayer attack. So the obvious slayer advantage has to be taken into consideration. Apart from that, it was probably Goumon from Avatar, whom he rescued Lucy from in the dungeon.
Gray, however, did froze a half-powered Mercuphobia's laser beam.
And I highly doubt any of Gajeel's ranged attacks possess the same potency as a laser beam from Mercuphobia, even when he was at half of his full power. So I think Gray can and should freeze some of Gajeel's spells, probably the roar and other projectile attacks that he expels from his body, but not really the physical ones. And most certainly not Gajeel's entire body.