The 19-year-old former Celtic youth team player opened the scoring with a well-taken goal while his overall performance caught the eye and earned praise from the Ton assistant boss.
McNally, in charge in the dugout with Allan Moore taking the opportunity to spy on Saturday's opponents Cowdenbeath, said: "Liam Gormley has been in training with us. He looked sharp in the box, particularly in the first half.
"He's not played for a while and was nowhere near match fit, but getting the full 90 minutes today will bring him on in that respect.
"He's scored and you've got to be in those positions to get them. He's got that about him and you can't coach it.
"It was a good finish today, he's got a wee trick or two and comes and takes the ball from the midfield.
"We'll want to see a bit more of him before making a final decision and the
upcoming reserve matches will be ideal for giving us a chance to judge him against that level of opposition."
Ton were invited down the coast to provide the opposition as the junior side unveiled their new 3G astroturf pitch, donated by Euromillions winners Colin and Chris Weir, and ran out comfortable victors.
Gormley got the ball rolling on 19 minutes when he controlled a Jordan Halsman shot, turned and slotted under veteran Largs goalkeeper Stephen Connick.
The frontman continued to trouble the hosts and won a penalty on the stroke of half-time after Ian Fisher bundled him over inside the box. Stephen Stirling assumed spot-kick responsibilities and confidently stroked home goal number two.
Largs, who had lost 4-0 to Kello Rovers on league duty the previous day, made five changes at the break.
Morton, meanwhile, made one switch, replacing Archie Campbell with Dylan McLaughlin after the skipper for the day had become embroiled in a heated exchange with home defender Barry McLaughlin - a player on Moore's radar just over a year ago - which threatened to boil over.
Morton's McLaughlin looked lively and created the third goal by wriggling away from a defender on the right before firing over a low cross which Tony Wallace slammed past Connick from 15 yards.
Substitute Aidan Fulton joined the fray with quarter of an hour remaining and grabbed the goal of the game five minutes after his arrival.
The left midfielder met Stirling's incisive pass in mid-air and sent a dipping strike sailing beyond Connick's despairing dive and into the net.
Two minutes later, Largs trialist Curtis Rose netted a consolation goal when he rounded off a quick counter with a clinical finish. On what was a day of celebration for Theesel, it was fitting that they had the final word.
Morton (4-4-2): Hutton (6) (McFarlane 74, 3); Boyd (7), Naismith (6) (Blair 68, 3), O'Ware (7) (Maguire 79, 2), Halsman (6) (Keogh 70, 5); Wallace (6) (Frizzell 79, 2), Stirling (7) (McLean 79, 2), Wilkie (6) (Lynch 79, 2), McDaid (7) (Fulton 74, 7); Campbell (5) (McLaughlin 46, 7), Gormley (7).
l Morton reserves kick off their league season on Wednesday night with a game against Ayr United at Cappielow. Kick-off is 7.30pm.