The 32-year-old, who has had spells with the Saints, Ross County, Partick Thistle and Plymouth in recent years, has been without a club since being released by Carlisle at the end of last season.
The defender, who can also play as a defensive midfielder, has plenty of experience with over 300 appearances and has played in the Scottish Premiership and League Two in England.
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Ray McKinnon, said: “We are really pleased to secure Gary’s services. He has been in training with us for a week or so and we have been really impressed with him. He seems to be really enjoying it too and has settled into the group well.
“Adding real experience to the squad only enhances the group. In Gary we have an excellent pro and a player who can feature in a number of positions.
“Providing we can get international clearance through in time, we are hoping to have Gary involved in some form tomorrow night.”