Only the following iPhones running iOS 7 or higher, can install the giffgaff carrier settings. Theses settings cannot be installed on any other iPhones or iPads:
iPhone 7 Plus
iPhone 6s Plus
iPhone 6 Plus
If your iPhone is running iOS 7 or higher then you will receive a notification telling you that a carrier settings update is available. Simply follow the on-screen instructions to install the giffgaff carrier profile.
If your iPhone is running an older version of iOS, you will have to update it to the latest version to install the carrier settings. iPhones that cannot update past iOS 6 will need to manually install these settings using this guide.
If you haven't received the notification, you can force your iPhone to check for the update by using a wifi or other Internet connection and going to:
Wait a few seconds and the phone should prompt that an update is available.
Alternatively, you can install the carrier profile with iTunes when you connect your iPhone to your computer and then check for updates.
If you experience any difficulties installing this update please read this useful article: "iPhone carrier file tips" which also has a link to a dedicated thread for reporting any problems with using this new carrier file.