"This is a difficult and emotional day for me," the 34-year-old midfielder told a news conference at Atletico's Wanda Metropolitano stadium.
"It's been the hardest moment of my career. This club is a lifestyle for me."
Gabi made 417 appearances for Atletico and also played for La Liga rivals Getafe for one season on loan and Real Zaragoza from 2007-11 before returning to the Spanish capital.
(Reporting by Arnab Paul in Bengaluru; Editing by John O'Brien)
