4G will be available on giffgaff later this year.
An I Phone 5 wont be able to get Giff Gaff 4G as the bands O2 the network Giff Gaff use isnt suitable.
ANyway Giff Gaff 4G is a few months away.
An I Phone 5S or 5C would work on 4G when it comes.
Hi, check out the following link:
4G is coming soon to giffgaff!