You can try iCab Mobile. It works with most (but not all) sites.
The better solution (when available) is to get the app for that site. Gumtree has an app called MyGumtree. It's for iPhone but should work on the iPad (if not prettily). Reviews are poor, but the app is free so you might as well give it a try.
On the other hand, the Craigslist app is awesome.