McGowan, whose only European Tour title came at the Madrid Masters two years ago, had 10 birdies and two bogeys in carding an 8-under-par 64.
Gonzalo Fernandez-Castano, who also strung together five consecutive birdies, is one stroke back at 7-under. He's tied with Lorenzo Gagli, who birdied his final three holes, and Brett Rumford for second at 65.
Edoardo Molinari is alone in fifth at 6-under with a crowd of nine golfers tied at 5-under.
McGowan opened with birdies on his first two holes and was 3-under after four. A bogey at the sixth hole dropped him back before he ran off five straight birdies over Nos. 9-13.
He had another bogey at 15 but birdies at 16 and 18 left him alone in the lead. He hit his approach on in the 577-yard, par-5 to within 2 feet for an easy putt for birdie.