Last season, Arsenal triumphed 1-0 at Old Trafford before taking the spoils with a 2-1 victory at the Emirates.
"Gallas is a monster as a footballer and is an old friend so I listened to him.
"William told me to stay cool and never let my head go down. He also said to always fix him with a look straight in the eyes and to stick my chest out in pride.
"He reminded me that football is hugely down to making an impression on the guy you are marking, dominating him if possible.
"It's a proud memory of mine that long before the end of the game Ronaldo had switched wings.
"In my view, Gallas had helped me win my duel with United's star player."
Arsenal are the early pacesetters in the Premier League this year.
Clichy added: "In previous seasons, there was a little bit of a split with the youngsters on one side and the older players on the other.
"Nowadays there's no longer any separation."