Gretna stretched their lead at the top of the second division to 10 points in a top-of-the- table clash that exploded into life late on.
The first half was goal-less as Morton concentrated on restricting Grena's chances without creating many themselves.
Kenny Deuchar, who celebrated his 50th goal for the league leaders against Wraith in midweek, should have put Gretna in front in the 15th minute when he shot tamely at Morton keeper David McGurn.
Kevin Finlayson had Morton's best chance of the half after 37 minutes when he shot into the side netting when well placed.
Gretna went in front thanks to captain Chris Innes in the 57th minute. David McGregor upended flying winger Ryan Baldacchino and was booked. Gavin Skelton drove the free- kick into Morton's box, where Innes met it eight yards out to head home.
But it needed a superb double save by home keeper Alan Main to preserve Gretna's lead in the 67th minute. Andy McLaren burst past Innes and the Gretna keeper blocked his shot, which went straight back into the path of Derek Lilley. He then kept out Lilley's drive with an outstretched leg.
With three minutes of ordinary time left, David Bingham set up Ryan McGuffie to double Gretna's lead from close range, but Morton fought back with a scorching 25-yard drive by Scott McLaughlin after 90 minutes.
The match was sewn up three minutes into time added on when Baldacchino collected a clearance from David McGurn to score with a faultless 40-yard lob.
McGuffie then hit the Morton bar, but it would have been rough justice on the visitors had they had lost by a bigger margin.