Gallas is in line to make his Spurs debut today against West Bromwich Albion.
Redknapp reckons Gallas has proven he has mental strength simply by agreeing to move to White Hart Lane.
"You have to be strong as a person to do that - it is easy not to have done that," he said.
"He could have gone off and played somewhere else."
Redknapp believes a player of Gallas' calibre would have cost up to £10million on the open market.
"If you want a player who is a top centre-half to play in the Premier League, you have to play that money, surely," he said.
"I think he will be a great signing for us. For me, he is a no-brainer. He cost nothing. He is not on silly money."