'Gone Aways' are those mailings that are undeliverable by the Royal Mail. If you have a "sender" address on your mailings, the Royal Mail will return any undelivered mail to you and provide a reason for non-delivery.
Experian class the following reasons for a non-delivery as a 'Gone Away':
Other reasons may also be provided such as 'refused' but these are not accepted by Experian as a 'Gone Away' as the address can be delivered to.