You can get an early Fairy Tail 319 spoilers of the manga every Wednesday while the Fairy Tail 319 scans English version were out on Thursday.
It does seem plausible that this ability is a first gen DS only, because unless I missed a chapter or can’t remember it, we weren’t told by anyone with extensive knowledge of DS magic what would happen if a DS ate a dragon (?) element other than his own. All we was told is that DS can eat their own element to recover their magic and the only other things Natsu have eaten other than fire was a physical item (lacrima) and lightning.
Imp of Raven Tail? Could be, since Gajeel kinda betrayed RT and they would most likely gonna get him pay, why not make Rogue kill him?. Former master of Phantom Lord, Joze? Same idea, he could become an evil guy and operate from shadows, his magic was also shadow like. He has a reason to hate Gajeel, since he joined FT. (I rewatched phantom lords arc lately, thats why I’ve got this wild guess).
What is given is this substance probably hates Gajeel, it is unknown for Mavis but seems to me its familiar to Jerar and co., so it might be something big or small. Anyway Mashima knows how to make mistery. Good chaper.