To change the location of your ad simply click the "My Gumtree" tab and then click "edit" option for your ad if you are registered.
You will be taken to "edit ad page" of your posted ad and here you will need to scroll down to available location droplist. Change the location by choosing from the available options and then click on "Post Your Ad".
If you are not registered refer back to the confirmation email you would've received after posting your ad and click on the available "edit" link in that email. This link will take you to the "edit ad page" where you can select from the available locations and then click on "Post your ad"