Diego Simeone's side, who play Spanish rivals Athletic Bilbao in the Europa League final on May 9, stay sixth on 50 points with two matches left.
They are two behind fifth-placed Levante ahead of the Valencia-based club's match at Real Zaragoza.
Levante are three behind Valencia and Malaga in third and fourth respectively, which carry an automatic Champions League berth and a qualifying spot for Europe's elite club competition.
Gabi fired Atletico ahead in the 54th minute at the Calderon when he made space just outside the penalty area and drove a powerful low shot into the corner.
He then picked up two yellow cards in five minutes, the second a somewhat harsh caution for an innocuous-looking high foot.
Sociedad, who are 13th, snatched an equaliser in added time when the Atletico defence was too slow to clear its lines and substitute Carlos Vela scrambled the ball in.