The 43-year-old actress, dressed all in black, stepped out at a ceremony marking the 75th anniversary of a famous wartime address by General Charles de Gaulle.
Hollande, 60, split with his long-term girlfriend Valerie Trierweiler last year after revelations that he was having an affair with Gayet.
Arriving shortly before Hollande at the ceremony at Mont Valerien, on the western edge of Paris, Gayet was seen pushing the wheelchair of her grandfather Alain Gayet, a decorated hero of the French resistance.
The ceremony marked the anniversary of De Gaulle's historic radio address from London on June 18, 1940, urging French citizens to resist German occupation.
Accompanied by her father Brice, Gayet stayed discreetly at the back throughout as Hollande shook hands with the former resistance fighters.
The presidency insisted it was a coincidence that Gayet was at the event. "There was no strategy on our part," an advisor to Hollande said, saying the actress attended the ceremony in her capacity as the granddaughter of a decorated resistance fighter.
The surprise appearance came as society magazine Paris Match splashed the headline "Julie Gayet comes into the light" on its front cover, reporting Thursday that the actress is set to "enter her new role little by little" as Hollande's official partner.
In April, celebrity magazine Voici printed snaps of the two walking together at La Lanterne, the president's residence at Versaille outside Paris, alongside some old friends of Hollande.
The same magazine had in November published pictures of the two looking cosy together inside the Elysee presidential residence, prompting five staff members to be reassigned over what constituted a damaging security breach.
Repeatedly asked in recent months about the status of his relationship with Gayet amid huge speculation in the tabloids, Hollande has neither confirmed nor denied that the pair are still an item, saying merely that he has a right to have his privacy respected.
Although we can probably rule out a wedding even in the pair do go public. At the marriage of former French finance minister Pierre Moscovici last week, Hollande is reported to have told guests that he is too old to get married and that it costs too much.
Paparazzi photos in January 2014 of Hollande in a scooter helmet, visiting Gayet at a Paris apartment, set off an international media firestorm.
Trierweiler, a journalist, published a bestselling tell-all memoir of their subsequent break-up, portraying the president as a cold-hearted egotist and champagne socialist, who was contemptuous of the "toothless" poor.
The president has four children with another former partner, the current ecology minister Segolene Royal.