Wayne Gretzky allegedly lied about dropping pounds by chewing gum -- and a man is now suing the hockey legend over it ... claiming The Great One's great fib cost him $10 MILLION.

The allegations are all spelled out in a new lawsuit, obtained by TMZ Sports, filed in Los Angeles on Monday.\

The plaintiff, Steven Sparks, says he created a "natural gum used to manage weight," and called it "OMG gum." He says in the suit he eventually hired on Wayne's wife, Janet Marie Gretzky, to be a spokesperson for the product.