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First, please note that you cannot use a goodybag with unlimited data on your iPad. This is strictly forbidden and your SIM will be blocked. Any other goodybag/a gigabag/normal credit is fine though.
First, Go to Settings, then WiFi, and turn WiFi off. This enables the internet settings to be successfully input.
Now, go to Settings again, then Cellular (if available), then Cellular/Mobile Data, and make sure Cellular/Mobile Data is switched on.
Then choose Cellular/Mobile Data Network. In the boxes, type:
Now power off your iPad and switch it on again. The internet settings are now complete.
N.B. If the Cellular/Mobile Data tab does not appear, see this guide:http://www.bit.ly/iphonenocellulardata
Your iPad will say O2-UK as the carrier, because giffgaff uses O2's signal. If you would like to change the text to giffgaff, there's a long winded way of doing this which requires jailbreaking. PM me if interested.
Also, the View Account button in Cellular/Mobile data will not work, but simply log onto your giffgaff username on the website to find out how many MB you have left.
Screenshots courtesy of darrenpainter.
EDIT: Have added bit.ly link in case anyone wants to link to this guide: http://bit.ly/giffgaffipadsettings