Blaugrana entered the match knowing that they needed to win to remain in the La Liga title race, as leaders Real Madrid had beaten Cadiz 3-0 earlier in the afternoon.
Xavi's side led 1-0 and 2-1 against Girona but were ultimately beaten 4-2, with the result handing the championship to Real Madrid with four games to spare.
Barcelona have also now dropped to third spot in the table, one point behind Girona, and it proved to be a third defeat in the team's last four matches in all competitions.
Xavi said that the defeat showed "a lack of maturity", with the club's legendary former midfielder showing his frustration following the full-time whistle.
Barcelona are now battling for second spot with Girona
"It is hard to explain this. This is the summary of our season, we gifted them the match," Marca quoted Xavi as saying after the Catalan derby.
"We dominated the game, and we had it under control. We could not score a third goal and then we gifted three.
"This match shows a lack of competitiveness, a lack of maturity, and individual errors that should not happen at this level.
"Last season we reduced our individual errors, but this season we doubled them. They scored two goals in two minutes because of our mistakes.
"Michel told me he could not believe how they won today. We gift the matches to our opponents. It happened twice vs Madrid, now against Girona. Even against Paris (Paris Saint-Germain)."
Xavi sends congratulations to Real Madrid
Xavi also congratulated Real Madrid on their title success, with the capital outfit proving to be comfortably the best side in Spain's top flight during the 2023-24 campaign.
"Real Madrid won the most points, so yes. Congratulations to them," he added.
Barcelona now have league matches to come against Real Sociedad, Almeria, Rayo Vallecano and Sevilla to finish their 2023-24 campaign.
Xavi had been due to stand down as head coach this summer, with the 44-year-old announcing in January that he would be leaving at the end of the campaign.
However, it was recently announced that Xavi would now be staying heading into next season, and the club are currently planning for what is expected to be a busy transfer window both in terms of incomings and outgoings.