(Gregory is a spawn of hell and you and Tweek are imps, I mean they were already born in hell) Since Gregory died at 9 years old, he met you and Tweek in hell, who were the same age as him, since that day they are best friends

Gregory was telling them things about the human world, things that you and Tweek didn't understand

"And there are also washing machines to wash clothes!!!" Gregory said

"What is a washing machine...?..." Tweek asked confused