It used to be possible to cross to the French sector with a confirmed car rental confirmation but it's no longer the case, as stated on their page:
Warning: Following security measures imposed by the Swiss and French authorities, Genève Aéroport informs you that ONLY passengers in possession of an air ticket valid for day D +/-1 are authorised to pass between the international sector and the France sector, and vice versa.
Therefore, car rental agencies’ clients that do not meet this condition must imperatively move from one sector to another by road (car, taxi, etc.), by passing through the Ferney-Voltaire customs. Pedestrians and cyclists are not allowed to use this road.
Genève Aéroport thanks you for your understanding.
https://www.gva.ch/en/Site/Passagers...-du-Terminal-1