The Gold Leaf key purpose is to open the right exit of 'The Broken Common' area. This zone is located north of the zone with a Bush Buck Coffee Stand. Once you open the area with the key you have a new puzzle to complete to get a page.
As for getting the key i don't remember how you get it, maybe it's purchasable a the Jungle Camp or with better change when solving puzzle and freeing an animal, sometimes you get a key.