If you pay for Disney's transportation for you and your guests into the dessert party (approx. $500) then yes, you can wear your wedding gown. If you do not pay for their transportation and you and your guests simply walk into the park on their own and proceed to the location of your dessert party, then you ARE NOT permitted to wear your gown.
The rationale for this is that they do not want you walking through the park in your wedding gown. Their transportation will take you into World Showcase through a back entrance to your dessert party location. That way, no one sees you in the park in your wedding gown and mistakes you for a princess--even though you will probably look like one!