If you've used up your data allowance before your goodybag or gigabag expires, GG will send a notification to your phone to let you know. You will need to top up your credit in your account to carry on using data, if you have, then you will be charged at 2p/MB for the life of your goodybag, but once your goodybag expires, standard payg rates will apply if you don't buy a new one.
Mobile Internet is charged at 20p a day (up to 20MB).If you use between 1MB and 20MB you will only be charged 20p/day. If you go over 20MB you'll be charged extra at 20 per MB.
If you text the word "plan" to 43430, you will get a text back from giffgaff on your usage for the last 30 days.