Trippier won the LaLiga with Atletico before returning to England at Newcastle United.
Former Chelsea midfielder Gallagher was introduced to fans at the Metropolitano yesterday and spoke to the club's TV channel about the move.
Metropolitano: "It's an incredible stadium. It's my first time here and it's just huge. And it makes me even more excited to go out and play."
His qualities: "I try to do a little bit of everything on the field. I always work as hard as possible. I leave everything on the field. I try to create scoring options and I also help the team as much as I can in defense. So, like I said I try to do a little bit of everything."
Trippier: "Firstly, Kieran Trippier is a great player and an even better person. I spoke to him a lot about the club and what he thought and he only said good things to me. He said how incredible the club is, the fans, the city and that helped me make the decision."
His Atleti objectives: "I want to help the club as much as possible. Obviously, the objective is to win the title. And of course, I am here to win titles. It is a very big club that has already won many titles and I hope we can win some more together.'
On former Chelsea teammate Cesar Azpilicueta: "I have a good relationship with Azpi. I spoke to him and he only said good things about the club and why I should come."