Gardiner joined the SSFC academy in the summer of 2020 and excelled during his short time in claret and blue.
After a successful eight-week trial, Gardiner has signed a two-and-a-half-year scholarship with Sunderland.
He told South Shields: “I’ve absolutely loved it at South Shields.
“It’s a family club with so much togetherness running through it.
“The coaches at both academy and first-team level were great with me, as were all of my team-mates.
“It’s a great club and I’ll definitely get to as many games as I can in the future and keep in touch as I loved every minute of it.
“You always knew there was a pathway to the first-team too because the first-team coaching staff would regularly watch us train, which always gave us that added boost.
“I feel like I’ve improved so much as a player as a result of my time at South Shields, particularly with my knowledge of the game, and I can’t thank everyone at the club enough for making my time there so valuable and enjoyable.
“I’m excited to start my journey with Sunderland as they were my boyhood club and to now get the opportunity to play for them is a special moment for me.”