I thought I have replied you earlier. As far as I know, giffgaff doesnt have roaming partners in the Canary Islands so I think its not going to work. Best bet would be to get a local SIM there
You will be charged on roaming rates, (goodybag allowances do not apply)
Please ensure you have sufficient credit maybe, 20 pounds or so.
If you do run out, you always topup online on giffgaff.com
Try selecting the network manually
You can see a list of giffgaff roaming partners here
I WILL BE STAYING 7 DAYS in canary island, AND MAINLY INTERESTED IN USING FACEBOOK, INSTAGRAM ETC ANY IDEAS HOW MUCH I SHOULD TOP UP TO LAST ME THE WHOLE WEEK?
If you plan you use data whilst roaming, be prepared to pay 45p/MB. You should seek out WiFi where possible to keep costs down. How much credit you will need will depend on how much you use, so try tracking your daily usage before you go, multiply each MB by 45p and you should get an estimate of how much you might need.