Make sure you have "private browsing" turned off on your "safari browser", as this will allow the site to load properly and you'll be happily browsing Gumtree once more.
1) Open Safari, click the Bookmark icon (looks like an open book).
2) Click "Private"in the bottom left corner.
3) Specify whether or not you'd like to keep all your current tabs open for private browsing (close them if you've got Gumtree open!)
4) Click "Done" in the top right corner.