Lewis Hamilton topped the times ahead of team mate Valtteri Bottas as Mercedes dominated Friday morning's opening free practice for the French Grand Prix.
The defending five-time world champion and current series leader clocked a best lap in one minute and 32.738 seconds to outpace the Finn by just 0.069 seconds on a hot day at the Le Castellet circuit.
Charles Leclerc was third for Ferrari, more than three-tenths off the leaders' pace ahead, of Max Verstappen of Red Bull, four-time champion Sebastian Vettel in the second Ferrari and Pierre Galsy in the second Red Bull.
The session was largely uneventful save for spins by Verstappen, Gasly and Daniil Kvyat of Toro Rosso as the drivers struggled for grip.
British rookie Lando Norris was seventh for McLaren ahead of team-mate Carlos Sainz, Daniel Ricciardo in a Renault powered by a new upgraded engine and Alex Albon of Toro Rosso.
Fresh from his arrival overnight after attending a memorial event for fashion designer Karl Lagerfield in Paris on Thursday, Hamilton resumed normal service almost immediately.
He won last year's race and with a 29-point lead ahead of Bottas this season will seek to build on that advantage in Sunday's race at the remote circuit in the Var countryside.
Bottas, however, without a win in three races since the Azerbaijan Grand Prix, has claimed he is capable of rallying to challenge the Briton.
"The points gap to Lewis - it's still manageable," he said. "It's 14 races to go and that's a lot of points so I just need to try and get those wins now and get the big points.
"It's going to be about small margins and taking the opportunities on each race weekend."
After taking a new Honda engine, Kvyat is set to start from the back of the grid for Toro Rosso, having exceeded the number of power-units allowed by the regulations.
Both Red Bull drivers also took new Honda engines but remained within the permitted number of unit changes.
Ricciardo was the only Renault driver fitted with a new engine, the team deciding that Nico Hulkenberg should wait for the upgrade at a later race because he would otherwise have taken a grid penalty.
The team did not want to see him demoted on the grid at their home French race.
Team boss Cyril Abiteboul said: "Nico will take his new engine at a different race, but as he will incur a penalty for exceeding the three ICEs permitted per year, we don't want it to be in France, our home race."