In the first competitive session of the new season, the French racer completed a qualifying simulation with 12 minutes of the hour remaining to the head the timing screens.
Shod with the red-walled C3 Pirelli on the new 18-inch wheels, Gasly posted a 1m34.193s benchmark to end the daytime running fastest over Charles Leclerc by three tenths.
That compares to the Bahrain pre-season test-topping 1m31.720s set by Max Verstappen on the softer and faster C5 tyre, which is not open to selection this weekend. Meanwhile, the Dutch racer also landed pole for the 2021 GP at Sakhir with a 1m28.997s.
Charles Leclerc was the first to venture out on track - his Ferrari F1-75 sporting a sizeable aero rake across the rear - and he led a queue in the pitlane ahead of Valtteri Bottas.
But as nine of the 20 cars immediately hit the circuit, Bottas was forced to cut short his run in the Alfa Romeo when he reported a misfire and cruised back to the pits.
Leclerc used the slowest available C1 tyres to initially set the pace with an unrepresentative 1m45.601s but was immediately shuffled back by a gaggle of cars.
That left the two soft tyre-running McLarens to ascend the leaderboard as Lando Norris ran to a 1m36.584s to pip his returning team-mate Daniel Ricciardo by 0.3s after the Australian tested positive for COVID-19 during the pre-season test in the Gulf country.
But the session was soon after red flagged to retrieve debris on the main straight that had been deposited by Esteban Ocon after the sidepod bodywork on his Alpine ruptured.
That failure after seven minutes showered the pursuing Aston Martin of Lance Stroll in carbon fibre, while his temporary Aston Martin team-mate Nico Hulkenberg - subbing for Sebastian Vettel after his COVID brush - battled locking up the fronts into Turn 8.
Once the marshals cleared the track after a 13-minute delay, the second Alpine of Fernando Alonso returned to the pitlane first for a practice start ahead of Ferrari’s Carlos Sainz Jr.
Alonso was the first driver to knock the McLarens off their perch as he ran the quickest C3 soft tyres to a 1m35.247s effort to find 1.3s over Norris before Verstappen used medium C2 rubber to find another 0.4s and climbed to the stop of the times with his 1m34.783s lap.
Verstappen swiftly improved by another four hundredths on the same tyre, working his way down to a 1m34.742s as Red Bull stablemate Sergio Perez ran 0.3s adrift in second on C2s.
A comparative lull in on-track action, with 17 cars returning to the pits, was ended with 19 minutes remaining as teams largely switched over to their qualifying simulations.
George Russell emerged on the C3 softs, the Mercedes still struggling with porpoising after also shedding bodywork at Turn 1, and that afforded him the chance to go quickest on a 1m34.629s to fractionally move ahead of Verstappen by 0.1s with 15 minutes left.
Sainz overcame tripping over traffic caused by Kevin Magnussen’s Haas to bring the times down further and he was joined at the top by team-mate Leclerc - recovering from a spin at Turn 11 when he appeared to jump on the power too early - on the mediums.
But both Ferraris were immediately pipped by 0.3s thanks to the eventual 1m34.193s yardstick came courtesy of the soft-tyre run from Pierre Gasly.
From there on, few drivers improved to leave the medium-tyre pace of Leclerc and Sainz to occupy second and third ahead of Russell and Max Verstappen, who stuck to mediums in the latter part of the session.
Stroll ran to sixth fastest ahead of Lewis Hamilton in seventh, while Alonso and the second AlphaTauri of Yuki Tsunoda kept Perez at bay in the lower half of the top 10.
Rookie driver Guanyu Zhou ran to 11th ahead of Ocon and the Williams of Nicolas Latifi, while Hulkenberg’s substitute appearance started with 14th.
Alex Albon guided his Williams, which appeared to struggle with bouts of both understeer and oversteer, to 15th ahead of the two McLarens - Norris getting the better of Ricciardo.
Mick Schumacher edged the returning Kevin Magnussen for Haas, while Bottas never remerged from the pits in his stuttering Alfa Romeo to prop up the times.
F1 Bahrain GP - FP1 results
| Cla | Driver | Laps | Time | Gap | |
|---|---|---|---|---|---|
| 1 |  Pierre Gasly |  | 23 | 1'34.193 | |
| 2 |  Charles Leclerc |  | 22 | 1'34.557 | 0.364 |
| 3 |  Carlos Sainz Jr |  | 23 | 1'34.611 | 0.418 |
| 4 |  George Russell |  | 23 | 1'34.629 | 0.436 |
| 5 |  Max Verstappen |  | 22 | 1'34.742 | 0.549 |
| 6 |  Lance Stroll |  | 22 | 1'34.814 | 0.621 |
| 7 | Lewis Hamilton |  | 17 | 1'34.943 | 0.750 |
| 8 | Fernando Alonso |  | 14 | 1'35.000 | 0.807 |
| 9 |  Yuki Tsunoda |  | 20 | 1'35.028 | 0.835 |
| 10 |  Sergio Perez |  | 23 | 1'35.050 | 0.857 |
| 11 |  Guanyu Zhou |  | 20 | 1'35.053 | 0.860 |
| 12 | Esteban Ocon |  | 15 | 1'35.151 | 0.958 |
| 13 | Nicholas Latifi |  | 15 | 1'35.644 | 1.451 |
| 14 |  Nico Hulkenberg | | 23 | 1'35.815 | 1.622 |
| 15 |  Alex Albon |  | 17 | 1'35.923 | 1.730 |
| 16 | Lando Norris |  | 22 | 1'36.304 | 2.111 |
| 17 |  Daniel Ricciardo |  | 20 | 1'36.402 | 2.209 |
| 18 | Mick Schumacher |  | 23 | 1'36.536 | 2.343 |
| 19 |  Kevin Magnussen | | 21 | 1'36.804 | 2.611 |
| 20 |  Valtteri Bottas |  | 2 | ||
| View full results | |||||
