Making a voice call or using data are both OK. Using data like googling for something is cheaper than making a phone call in HK (will cost about 1 pound).
I haven't tried sending SMS. You may try it - if it works, Giffgaff will send u an email to confirm that your sim card is safe and sound. If you haven't received Giffgaff's email after sending SMS for, say 2 hours, then just use some data.