Lewis revealed that it would take a lot of money to buy the New Zealander out of his contract with Wales, who face France in the semi-finals of the tournament on Saturday.
"Having worked in the music business for so long, when stars break and you sell millions of records and generate hundreds of millions of pounds, you have got to make sure you have got a contract there that is watertight," he said. "And we have got a watertight contract.
"Even if anyone wanted to come and have a conversation with us, and I am not suggesting this at all, it would cost them a heck of a lot of money."
Gatland took charge of the side in 2007 and guided them to the 2008 Six Nations championship.