You get 9GB at full speed then the speed drops significantly for the remaining of goody bag. If you exceed 20GB, you get charged for data usage.
There is only a single threshold -- though it depends what date you bought the goodybag.
9GB applies to goodybags bought before 11th July. 20GB applies to goodybags bought since 11th July.
When you exceed the threshold (whether it's 9 or 20GB, depending on age) you get an intentional speed restriction during the daytime but you do not get charged for additional data
The exception is if you take that goodybag outside the UK and use it in EU/EEA. Roaming partners are permitted to charge giffgaff 0.6p for the data provided while you are there. Giffgaff cannot afford to pay the roaming partner a potential unlimited amount (which could easily be more than you paid for the goodybag) so giffgaff and other networks are permitted under EU rules to pass that 0.6p on to you.
The simple answer is don't buy an always-on goodybag to take to EU -- save £5 and get the 20GB goodybag. Within the UK it's OK if you like having the additional speed restricted data, but not appropriate for everybody.