Los Che, who had collected just two points from a possible 15 since the middle of last month, picked up the win through a powerful Luciano Vietto strike in the second half.
The visitors, already assured of a Champions League spot, are now on 70 points for the season with one game - at home to Deportivo La Coruna - left to play.
A good chance came and went for Carlos Soler early in the second half, with the Spanish midfielder being denied from long range as Valencia continued to probe away for an opener.
The breakthrough strike arrived shortly after the hour, however, as Vietto was allowed to carry the ball and strike it into the top corner under little pressure.
GIRONA (5-3-2): Bono; Maffeo (Aday 69'), Ramalho, Bernardo, Juanpe, Mojica; Granell, Timor, Borja Garcia (Amagat 82'); Lozano (Olunga 73'), Stuani
VALENCIA (4-4-2): Jaume; Montoya, Vezo, Murillo, Latorre (Vidal 86'); Torres (Gaya 72'), Carlos Soler, Maksimovic, Guedes; Zaza (Rodrigo 71'), Vietto