Dream debut for Petrasso
Petrasso's second-half strike at Priestfield earned Latics their third straight victory in league and cup as they continued their recent improved form.
And the only disappointment for Latics was the late dismissal of defender Genseric Kusunga.
James Tarkowski could direct his header straight at keeper Stuart Nelson from a James Dayton corner.
The deadlock was broken moments later when Petrasso struck.
Danny Philliskirk cut in from the left and saw his shot well blocked by the legs of Nelson.
Jonson Clarke-Harris saw his follow-up shot blocked and Petrasso, following up at the far post, saw his header loop over Nelson.
Gillingham had a rare opening when striker Cody McDonald wriggled free but his angled shot was saved by the legs of Latics' keeper Mark Oxley.
Latics made their first substitution on the hour when Petrasso was replaced by Sidney Schmeltz.
There was a let off for Latics when Cody McDonald breezed past Tarkowski, but Oxley came to the rescue with a vital block.
There was a flashpoint with five minutes of normal time left when Kusunga was sent off for retaliating after a sything challenge from Danny Kedwell for which he was booked.
Ten-man Latics almost doubled their lead when Joseph Mills curled a free kick over the defensive wall as it flew the wrong side of the upright.
Full-time: Gillingham 0, Latics 1