Gallas was outstanding for Spurs this season and won a new contract.
He told L'Equipe: "I left Arsenal because we could not agree on a financial level.
"They decided to not make the effort to keep me, it is a choice I respect, and they respected the fact I didn't want the wages they proposed.
"Did Arsene Wenger make a mistake in not keeping me? He made a choice, but indeed I believe he made a mistake.
"In the street, Arsenal's fans asked me why I left. The coach thought I was a finished player. I proved he was wrong."