An option to repost an Ad will become available a few days before an Ad is set to expire. If you're registered you'll see the option in your "My Gumtree" if not you will receive an email and you simply need to click on the "Repost" link to bump your ad back up.
You can also simply post a new ad by copying the details of the existing Ad and posting a new version. Please delete your old ad before posting your new one.
Remember your ad will expire in 60 days in all categories.
So the good news is you don't have to wait till your ad is close to expiry, you can simply delete your current ad and post a new one anytime so that it's closer to the top of the category it's listed under.