Gallopin salvaged his difficult season with panache on the punchy 185.7km stage from Puerto Lumbreras to Pozo Alcón ahead of road world champion Peter Sagan (BORA-hansgrohe) and Alejandro Valverde (Movistar).
The Frenchman's 2018 woes included withdrawing from the Tour de France on Stage 12 as a valued Romain Bardet domestique.
"So many bad luck," Gallopin said after the stage about his season. "So many times I crash I was sick. So after I stop in the Tour, we had a plan I go to the Vuelta and now I win the stage so I think the plan was good."
"It’s fantastic," Gallopin said. "To win ahead of riders like Sagan or Valverde means something."
"The last 20 kilometres were really difficult. I stayed in a good position and waited for the right moment to get back to Herrada and counter-attack.
The AG2R Mondiale rider also signalled his ambitions at the Vuelta extended beyond stage success.
"Winning a stage at La Vuelta was part of the plan, it was one of the team’s goals. The idea is also to do good on the general classification but if there was an opportunity for a stage win, I had to seize it.
"I felt from the start of La Vuelta I have good legs. I’ve lost weight, I’ve trained in the mountains and there’s something to do with the general classification.
"I have nothing to lose, and if I hit the wall some day, then I’ll aim for another stage win.”
With his victory, Gallopin jumped two general classification spots to fifth overall, 59 seconds behind leader Rudy Molard.
The biggest general classifcation loser on the day was Team Sky's Michal Kwiatkowski who crashed at a critical moment on the stage and despite chasing hard could not rejoin the front group. He finished 30 seconds behind Gallopin.
Second to race leader Rudy Molard (Groupama-FDJ) before the stage, Kwiatkowski dropped four places on the general classification. Valverde moved up to second overall with Emanuel Buchmann (BORA-hansgrohe) third.
How the stage unfolded
Alexis Gougeard (AG2R), Michael Woods (EF Education First-Drapac), Floris De Tier (LottoNL-Jumbo), Nicola Conci (Trek-Segafredo), Edward Ravasi (UAE Team Emirates), Alex Aranburu (Caja Rural-Seguros RGA) and Oscar Rodriguez (Euskadi-Murias) were the men in the break with the seven riders building a lead touching 3min 30sec before the peloton picked up the pace.
De Tier took the KOM points ahead of Rodríguez and Gougeard at the Alto Collado de Laude 68.5km from the finish with Woods, who capitalised on a minor crash with 15km to go at the base of the rough road on the Alto De Ceal, the last rider in the break to hold off the Movistar led peloton.
The racing was dynamic in the final 10 kilometres with Nairo Quintana (Movistar) and Dan Martin (UAE Team Emirates) momentarily stopped by mechanicals but returned to the peloton.
Jesus Herrada (Cofidis) attacked with 5.5km to go as the counter-attacks came quick and fast from riders like George Bennett (LottoNL-Jumbo) and Quintana before Gallopin made the decisive move.
The 2018 La Vuelta a Espana - what's next?
Spain's grand tour continues with Stage 8, a 195km 'flat' stage from Linares to the uphill finale in Almadén. Just a a 10km, category three climb with a 3.4 per cent average gradient faces the peloton almost midway through the stage plus several rollers, so it may come together for a Peter Sagan style sprinter or puncheur. But considering the length, a breakaway might stay free and clear.