Offer walls are widgets that loads offers ads from an external ad network into a given website.
Offer wall crediting is done by the use of postback. The major difference between offers from offer walls and the other types of GPT ads is that the former ones are not "installed" on the GPT website, so their availability, implementation, crediting and tracking is done by an external source other than your local GPT software. Since each offer wall network has its own set of specifications, each offer wall must be individually installed on a GPT website and will require some custom coding to get postback listeners and crediting properly integrated. By default, GPTEngine includes the ability to create placeholders for a custom offer wall to be installed.