Sorry the iPhone5 is not comaptible with 4G on any network other then EE or 3.
Rushed design by Apple means that it only cover around half of the LTE (4G) frequencies in the UK and critically not the 800Mhz band used by giffgaff.
The phone cannot be modified.
For a fuller explanation please see this thread http://community.giffgaff.com/t5/Tips-Guides/Will-my-phone-work-on-4G-iPhone-5-iPad-3-Samsung-Galaxy...
4G lauches tomorrow on giffgaff BTW.