The six-time MotoGP world champion's departure from Honda was announced last month and the Spaniard was all smiles as he tried out the 2023 Desmosedici bike at the Ricardo Tormo circuit.
The Spaniard went top of the timesheets with an hour and a half remaining before Aprilia's Maverick Vinales (28) went 0.171 seconds quicker, with KTM's Brad Binder (28) ultimately second and VR46 Ducati's Marco Bezzecchi (25) third.
"Today was amazing, it was a really nice day... the feeling is great on both sides of the garage; about Marc I cannot speak too much but the lap time you can see," said Gresini team manager Michele Masini.
"We are a bit tired after this long season but we can't wait to start the new season with this feeling... with this atmosphere we can work really well."
Double world champion Francesco Bagnaia (26) was 11th for the Ducati works team after trying out a new chassis and completing 51 laps.
Luca Marini (26), Marquez's replacement at Repsol Honda, got down to work with his new team after leaving VR46.
The first official test of the 2024 season is at Malaysia's Sepang circuit in February.
