As you would be roaming outside of the EU, you CANNOT use any of your goodybag allowaces (calls / texts / data).
To use your giffgaff SIM outside the EU you must purchase separate ‘credit’ here
You should also check the ROAMING CHARGES
Note that data roaming charges can build up quickly (particularly in places where it can cost £5 per MB). If you need to use the phone regularly for data, I highly recommend you buy a local SIM card as it would almost certainly work out much cheaper for you.