The 20-year-old, who spent four months on loan at the club last season, will undergo a medical on Monday ahead of a move.
Pirates boss Mark McGhee told Sky Sports News: "We are delighted to be bringing Tom to the club. He is a left-sided centre-half which will give us great balance playing alongside either Garry Kenneth or Adam Virgo.
"He proved last season that he can play as well, and he will be a great addition to strengthen our defence."
Parkes has 18 England Under-17 caps to his name.