Assuming you are not subject to a data bar (see your incoming messages from agents on your giffgaff account homepage) and your goodybag is live, as you mentioned:
1. Check if the issue only happens in one location:
Please verify whether the issue happens in just one location or not. If it does only happen in one location then please see point 3. If it happens everywhere you go then please see point 2.
This link will tell you where your nearest mobile phone masts are:
2. Test your SIM in another phone
If an issue is happening everywhere then it will almost certainly be caused by the phone or SIM card. The best way to check this is by testing the SIM in another phone. If the issue follows the SIM card, then you'll be able to pick up a new one from giffgaff here:
if it turns out to be your phone, perhaps there is something wrong with it you could Google or worse case, try a factory reset (be careful with this as you may lose data!)
3. Check your coverage
and the state of the network:
If your coverage should be good, then please see point 4.
4. Check the giffgaff service updates:
failing all that (and always a good idea)
5. Check for existing threads
Have a gander at other threads on the board to see if anyone else is experiencing the same issue.
Hope you're back up and running properly soon!