Nikko-sama!!!
Is Gabriel Sam's reincarnation?
I think that Sam's soul reincarnated as Gabriel in his new life while also parts of his soul are owned by Virgil, this could be why Gabriel is a bit dumb dumb in the head.
Cause in my religious beliefs that the soul have three parts and six something, that if you loses even 1 parts of it you're mentality undeveloped something.
But anyway, Gabriel is very similar to Sam, his behaviours and stuffs. Also often times how Sam took over Gabriel identity for moment. And dues to what he said we knew Gabriel is possessed by Sam. But I just had a feeling it's more than just possession by different soul but it's the reincarnation of said soul itself.