The French winger, a summer signing from Luton Town, made an impressive impact as he used his pace and power to set up his side's first goal before catching out Ton's trialist goalkeeper with an opportunistic strike to seal the win.
Despite being two weeks behind Ton in terms of preparation for the coming season, Walsall made an energetic start and twice went close in the opening 15 minutes. Jamie Paterson was instrumental on both occasions as he sent a powerful volley a foot over the crossbar in the 10th minute, and then picked out George Bowerman with a pin-point cross five minutes later.
Bowerman surprised Colin Stewart by immediately hooking the centre towards goal, but the Ton keeper scrambled across goal to claw the ball off the line before pushing it round the post for a corner.
Morton weathered the early storm and began to assert themselves, with Andy Jackson causing the Walsall rearguard problems with his movement and relentless harrying.
In the 17th minute, the former St Johnstone man chased down a Stewart clearance, causing panic in the Walsall defensive ranks.
The ball eventually broke invitingly for Peter MacDonald, whose cute lofted effort sailed agonisingly wide.
Walsall trialist David Grof, who observant Ton fans might remember as the sub keeper for Hibs in their side's 4-3 League Cup win three years ago, was finally forced into action in the 26th minute, lunging to his right to flip Jackson's glancing header away from goal.
Grof came up trumps again less than 60 seconds later, dropping to his right to block MacDonald's low strike, before denying the same player with another strong stop 10 minutes before the break.
Morton were in complete control at this point and really should have taken the lead in the 35th minute when MacDonald rolled the ball to Michael Tidser on the edge of the box.
The former Celtic youth elected to take a first-time shot with his weaker foot but then watched in disbelief as his placed effort rolled beyond the upright.
In true pre-season friendly tradition, both managers made a raft of substitutions at the break. Among Allan Moore's six replacements were trialists Keiran Brennan and an unnamed Italian goalkeeper.
New Walsall manager Dean Smith also made half a dozen changes - and it was those additions which turned the game in his side's favour.
Shortly after the restart, Adam Chambers burst down the right and whipped over a cross for fellow substitute Will Grigg, whose acrobatic volley clipped the crossbar.
Then, in the 62nd minute, Gnapka made an identical driving run to the byline before cutting back for the inrushing Chambers to sweep home despite the best attempts of Grant Evans to prevent the ball from crossing the line.
At the other end, Peter Weatherson was looking threatening in Ton's brief forays forward, testing Grof with a stinging low drive before crashing a powerful free-kick against the bar in the 78th minute.
But any threat of an equaliser was completely extinguished in the 86th minute when Gnapka rifled off an early shot from wide on the left.
The audacity of the effort caught Ton's poorly-positioned trialist keeper napping and bounced over his despairing dive and into the net to seal the win.