The barcode on a Giffgaff SIM holder is the SIM card number without the "894411" at the start.
Now updated for SIM numbers that begin 8944110068, 8944110066 or 8944110065
Just because it is on the internet does not mean it is magic. It only checks a couple of basic bits of maths in the numbers
It's probably got some bugs in it.
It's come from someone who, for all you know, is less trustworthy than a bloke you just met in a pub, who won't show his face, and whose real name you don't know, so if you do anything with any of the data from this page, it's you who will be held responsible for it.