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is your sim fully active?
see porting your number: http://giffgaff.com/support/transferask an agent for more help here: https://giffgaff.com/support/ask/transfer
most of the time this can be caused by something trivial like the date it has to go in exactly right eg 26/05/2015 or an incorrect pac code
If you have access to another browser try using that.. I find the GG website is more stable on Chrome than Explorer
Transfer you mobile number to giffgaff guide here will help you http://community.giffgaff.com/t5/Getting-started/How-can-I-transfer-my-mobile-number-to-giffgaff/ta-... All so retry it with with a different browser.