Theory:
Gamps is the main character from the future. I haven't played into Act 3 yet, but the things he knows are too spot on for him to have not experienced. Also him admitting to having the same abilities.
also the Artifact calls the MC the 25th him, which is why the beast is probably hunting the MC down. He is the 25th iteration, and the beast and artifact exist outside of time&space
The amount of times that the MC says "how did you know that?" rose this suspicion.
My other theory is that he's a god.
Or he's using Sawano to spy, but these theories aren't fun or lore heavy