This is what I'm wondering as well
When it was revealed the previous omega was Gray's right hand man, I figured he sent him out of jealousy. He was the previous fiance and was shown to have feelings for him. But El married someone else and the opportunity presented itself to put pressure on the marriage.
It also might've been to put a spy into the Duke's house, and the future mother of the heir no less.
But maybe it's the romantic in me that wants to give Gray the benefit of the doubt
could be both revenge and to steal mc, could also be revenge against the duke. He is a duke afterall close to the imperial and duke’s fam is shitty so they might helped in the issue of gray’s family
So… I’m guessing gray sent that omega to mc and duke’s house back then for revenge because mc’s family didn’t help his family. Mc’s mother and gray’s mother are friends but from the looks of it they didn’t help her