The Blues face off against the Catalan giants on Wednesday after the first leg between the two sides at Stamford Bridge ended off 1-1.
"At this time we were strong. When you play for a strong team, you don't have to speak about your opponent," Gallas told Goal. "We know, we knew there were really good. In the dressing room we didn't talk about them and say 'oh did you see Messi? He is really good.'
"Or 'oh, I am a little bit worried for you because you are going to mark him, so be careful.' No, no, because everybody knew what he had to do. At this time I was marking him and my team-mates knew I could do the job.
"I had all the information I needed from my manager [Jose Mourinho] and then I was focused to go on the pitch and do what I had to do. To be honest, I think I played maybe just two times against Lionel Messi, yes, he didn't score but the first time he got injured after just 15 minutes in the Camp Nou.
"For me, it was very important for me to compete against Messi, to know if I was a good defender. At this time Messi was still young but everyone was talking about Lionel Messi. I had seen some video of him and I thought, 'oh my god, I am going to mark him.'
"So I tried to find the best solution to mark him. That's why for the first 15 minutes I was very close to him because I knew I had the space to compete with him. What I knew as well was if I left him to get the ball and to run at me, I would have been in trouble.
"That's why the whole time I was very close to him. Unfortunately, he was injured after 15 minutes and I wanted to know that I could do it for 90 minutes. I played against him again with Arsenal but he was not playing in my area, so that's why I didn't have the opportunity to mark him.
"When I played for Chelsea, he was a right winger and I was a left back. When I played for Arsenal, I was a centre back and he was right winger. That's why I didn't have an opportunity to mark him. As a defender, when you play against players like him, you want to compete."