Shmeleva had been a warm favourite to complete her outstanding championships with another title to go with her individual sprint, team sprint and 500m time trial crowns.
Yet once the derny motorbike towing the field around moved aside, it was 19-year-old Gros who dominated the race, taking over with 350 metres left and resisting all attempts to outmuscle her in the final massed sprint for the line.
Belgian world champion Nicky Degrendele came on to her shoulder off the final bend but had to settle for silver while Shmeleva, who had never found the right position to attack until too late, won the bronze.
"I saw her (Degrendele's) wheel just behind me and when I passed the line I thought, 'Is it me? Am I European champion?'" Gros said. "Then I saw myself up there in first position and thought, 'Wow, it's me.' I couldn't believe it."
Great things have been expected of Gros ever since her triple gold medal-winning performance at last year's world junior championships in Montichiari, Italy, but this still marked a spectacular breakthrough.
The men's keirin, the last event in the programme, was won by Germany's former world sprint champion Stefan Boetticher who just prevented a French double by the narrowest of margins, edging out Sebastien Vigier for gold.
His victory meant Germany won more medals than any other nation in the track cycling events, 11 in total including three golds, while the Netherlands annexed the most golds with five, ahead of Britain's four.
The Dutch failed to add to their golden tally as double champion Kirsten Wild could only win bronze in the madison with Amy Pieters.
The Danish pair of Amalie Dideriksen, the 2016 world road race champion, and Julie Leth effectively grabbed victory by managing to steal a lap midway through the 30km race.
Matt Walls, at 20 another talented find for the British hosts, rode an immaculate elimination race, keeping out of trouble, picking off his rivals one by one and comfortably sprinting away from his final challenger, Portugal's Rui Oliveira, on the final back straight.
(Reporting by Ian Chadband, editing by Ed Osmond and Ken Ferris)
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