As you would be roaming outside of the EU, your goodybag is NOT valid.
To use your giffgaff SIM you must purchase separate ‘credit’ here
You should also check the ROAMING CHARGES
Note that data roaming charges can build up quickly (particularly in many places where it costs £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.
If you still want to use your giffgaff SIM in South America, the SIM should pick up signal automatically, and calls / texts should work. If your phone does not pick up signal, do a manual network search until it connects.
For mobile data abroad, you need ALL of the following phone settings. Check these before you travel:
• giffgaff APN settings, see this guide
• Data roaming turned on. Note this is a specific phone setting and is usually turned off by default.
• Mobile data turned on.