Hockey star Wayne Gretzky and movie actress Janet Jones are preparing to exchange vows in what is being described as Canada's version of a royal wedding. Gretzky, the Edmonton Oilers' center, and Jones, who starred in "American Anthem" and posed seminude for Playboy magazine, are to be married July 16 in a ceremony expected to attract celebrities from the worlds of hockey, show business and politics. The couple have found themselves in the center of as much excitement as that surrounding the Oilers' Stanley Cup victory last month. Seven hundred people have been invited; there will be 22 attendants, including a dozen bridesmaids and flower girls. Gift-givers, however, will have a tough time topping the groom's gift to the bride. "Do you want to know what he gave me?" Jones gushed. "It's a Corniche Rolls-Royce convertible, cream color with tan interior." The value? $250,000 or so.