The teenager finished well in the 13th minute following a through ball from Alexei Eremenko, who doubled the lead before the half-hour mark at Rugby Park on Saturday with a speculative effort.
Substitute Kenny McLean lifted St Mirren after the break, but his long-range strike against the post was the closest the visitors came.
St Mirren's lead over Hamilton at the foot of the table was only cut by one point to seven after Dundee United levelled after the break at New Douglas Park, but they saw Hibernian move six points above them.
The victory saw Kilmarnock bounce back from their 5-0 defeat by Aberdeen, their heaviest loss of the season by some distance.
Killie boss Mixu Paatelainen made five changes, with one enforced late on as Cammy Bell failed to shake off an ankle problem allowing Anssi Jaakkola to step in.
Jamie Hamill replaced Tim Clancy at right-back, Manuel Pascali and James Fowler added some steel to the midfield and Gros stepped in for Rui Miguel.
St Mirren, who had Michael Higdon in for the suspended Paul McGowan following his midweek winner off the bench against Motherwell, needed some desperate defending to avoid an early blow after Craig Bryson and then Eremenko drilled the ball into their goalmouth following a short corner.
Eremenko was dropping deep to dictate play and his dummy set up a slick break from Bryson and Mehdi Taouil but the Moroccan's final pass was just too far in front of Gros.
St Mirren defender Darren McGregor shot wide from 25 yards after an impressive run out of defence but Gros soon struck.
The 18-year-old laid the ball off to Eremenko, who threaded the ball down the left channel for Gros to run on to and squeeze a left-footed shot in off the far post.
Steven Thomson got a shot on target for Saints but Jaakkola saved easily and Hugh Murray resorted to a crude foul to stop Eremenko making another pass, earning himself a booking.
The Finland international put Killie two ahead in the 29th minute after Gros repaid the favour with an assist of his own, although Eremenko had St Mirren goalkeeper Paul Gallacher to thank too.
Gros laid the ball out to the right edge of the penalty area and Eremenko tried a speculative effort that bounced just in front of Gallacher and over his dive to nestle in the far corner.
Kilmarnock continued to look more dangerous in possession up the break.
Hamill almost got on the end of Garry Hay's arching cross and Taouil's brilliant pass over the Saints defence drifted inches too far in front of Fowler's run.
McLean, on for Jim Goodwin at the interval, instantly livened up the St Mirren midfield and almost got the visitors back in it in the early stages.
McLean collected the ball in the centre circle and beat a man to find space to strike a powerful low effort from 25 yards, but the ball bounced off the post and Craig Dargo was flagged offside as he eyed the rebound.
Killie were sitting deep but Eremenko wasted a chance to put St Mirren out of sight on the break when he stood waiting for Hay's cross to drop, allowing Jure Travner to nip in and clear.
But Saints came close again when McGregor met Travner's corner only for Bryson to clear his header off the line.
Eremenko somehow escaped a booking from Calum Murray for a two-footed lunge on Travner as St Mirren continued to press and the Finn then fouled McLean to give the visitors a chance to shoot from 30 yards, but Jaakkola easily saved from Higdon and had little to do in the final 20 minutes despite St Mirren's efforts.
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