The Pilgrims had trailed 2-1 at home to Southport, but came roaring back into contention courtesy of Tshimanga's 11th goal of the season on 80 minutes and McGowan's winner two minutes from time.
United had led through Ashley Hemmings' 22nd minute spot-kick but were losing at half-time courtesy of a quickfire Jason Gilchrist brace, the second of which arrived from the spot.