The 30-year-old central defender had one year left in his previous deal but the new agreement will keep him with the Florence club until June 2017.
"I am delighted," Gonzalo told accffiorentina.it. "There were never any doubts from my part.
"I had other offers but as soon as I was told that Fiorentina still wanted me, I immediately accepted."
Gonzalo joined Fiorentina in 2012 from Spanish outfit Villarreal and scored four goals in 33 league appearances to help the Viola finish fourth in Serie A.
"We want to improve with respect to last season," said Gonzalo.
"It would be fantastic if next season we can finish in the top three and qualify for the Champions League."
Gonzalo is hoping Colombia winger Juan Guillermo Cuadrado will remain at Fiorentina next season.
Cuadrado has been strongly linked with a move to Barcelona this summer while the 26-year-old is also a reported target of Bayern Munich and Arsenal.
"It depends only on Cuadrado," added Gonzalo. "He has so many offers.
"Juan is an intelligent person, who has given his all for Fiorentina.
"For us, he is obviously important as he is the best player in the world in his role.
"If he stays, we will all be very happy but it will be difficult to keep him."
Fiorentina, coached by Vincenzo Montella, will compete in next season's Europa League.