The 22-year-old, who scored seven goals for the Tynecastle club before being released at the end of the season, has joined on a one-year contract.
Manager Derek Adams told the club's official website: "Gary's a player who has always been highly rated by his coaches and has developed throughout his career.
"It is always tough for a young player to break in to the first-team having come through the youth set-up at big clubs, and I can understand his hunger to play competitive football.
"Having had the option to move to another SPL club, I am delighted he has seen Ross County as the right team and environment for him to make that step up in.
"We have a great track record of nurturing and developing young players in the first team here at Victoria Park and I think Gary's signing fits in well with that ethos."