When you are abroad, your device is automatically set up and will use the local mobile networks instead of giffgaff but you can still enjoy all of giffgaff services. If your using a goodybag, unfortunatley this wont work while you are away abroad as they are for UK use only. Instead you will have to top up with plenty of airtime credit.
Below is a guide to travelling abroad with your giffgaff mobile account.
And for pricing check here
New Roaming Price for EU
International calls arn't included in goodybags, therefor you would need credit.