Salt Lake dominated the first 75 minutes of the contest and had the lead thanks to a goal by Jamison Olave in the 7th minute.
Olave, however, brought down Justin Meram in the area and Mendoza's penalty kick tied it.
Gardner's goal came when his hard shot from 25 yards out ricocheted off Columbus defender Chris Schuler and drifted over the head of goalkeeper Nick Rimando with 8 minutes to play.
Columbus improved to 4-3-6 and Salt Lake fell to 6-3-2.
Swim Week Miami Beach 2014 [PHOTOS]