Looks like your phone is "locked" to a specific network, so you need to get it unlocked before you can use it on giffgaff or any other networks besides the one your brought it off.
Unlock codes can easily be brought off of ebay, or alternatively check this site: http://www.mobile-unlock-codes.co.uk – these tend to be cheaper options than using 3rd party high street retailers or even worse going to the network directly.
For a giffgaff sim with £5 FREE CREDIT when you activate, click here
Your phone is locked to Orange, call up Orange and ask for an unlock code. It may cost you, it may not, if it does take it to an unlock shop and get it unlocked for 10 pound and then you can use giffgaff.
If you like the sound of giffgaff why not Order a FREE sim card below that comes with £5 FREE credit!