Sergio Perez was second for Racing Point F1, seven tenths adrift at 1:17.842, while Ferrari ran into disaster despite Sebastian Vettel posting the third-quickest lap of the day at 1:18.195.
Ferrari later revealed that a mechanical problem caused the crash, but didn't provide any further details specifying the exact issue.
Leclerc, who joins Ferrari in only his second season in Formula 1 after a stellar debut for the affiliated Sauber team in 2018, said he was happy with his team's progress after recording a time of one minute 16.231 seconds.
"That's been the clear impression right from the first day and it hasn't changed".
Gasly, meanwhile, went into a spin at Turn 9, sending him off the track and into the barriers.
Williams remained the outsider as it continues to play catch-up for its late start to testing, but the team made progress nonetheless and finished the day with 140 laps for George Russell.
The Frenchman, who has moved up from Toro Rosso to replace departed Australian Daniel Ricciardo, was unhurt but the impact did considerable damage to the auto and ended the team's programme for the day.
Gasly ended up fourth, 0.007s slower than Lando Norris as McLaren's encouraging pre-season continued on the penultimate day.
Norris was one of three rookies to complete their testing program on Thursday, and another one of those - Alexander Albon - had a strong end to his running.
The lap was set on Pirelli's softest compound tyre, the C5, and is the fastest time in testing this year.
The works team is only eighth on the list of teams in terms of laps completed, with 289 - likely title rival Mercedes has racked up the most laps at 448. Especially after the lack of running yesterday.
Daniel Ricciardo in fifth was the highest-placed driver to set their best lap in the afternoon, posting a 1m17.204s.
Vettel was unable to return to action in the morning even after being cleared by the medical centre, as the team continued to look into the cause as well as starting to make repairs to the vehicle.
The crash meant Ferrari was unable to run for several hours on Wednesday and only covered a total of 41 laps.
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