Any calls made outside the UK will incur international roaming charges, and calls to international numbers from the UK will incur the relevant cost based on the prices shown at http://giffgaff.com/index/pricing
Labs prices only relate to UK Mobile/Landline calls as far as I am aware
I am trying Giff Gaff labs at the mo and also found it very confusing! I thought it was a one off price of £5 so was caught out last night when i ran out of credit! Surely i didnt use 500 texts in a week!
So now i know the deal, maybe it will still work out better than a goodybag but i think it should be explained better from thr beginning.