It won't let me use the data when the WiFi is turned off
Have you used data using this SIM before?
Do you have data enabled?
If you have used data before, then have you restarted your handset since buying the gigabag?
I have topped up with a £5 giga bagg do I need to use that then on the 9th of august I can start my goodybag?
Yes, a new goodybag can only be started when there current one is below 5 minutes or 5 MB of data remaining
If you bought the gigabag by mistake, and you haven't used any data (as you say youhaven't) then an agent may be able to remove it for you.
Please take a look here http://community.giffgaff.com/t5/Tips-Guides/CREDIT-AND-GOODYBAG-CONFUSION/m-p/988713#U988713 to see how credit and goodybags work.
I hope it helps you.
If in doubt, always ask first.
How do I contact an agent ?
Use this link http://giff.ly/agentscancelqueued
Just note, you're only able to use one goodybag/gigabag on your account at one time. When you queue a goodybag and want to start it before its expiry date, the balance has to be below 5 minutes ir 5 MB if data
As you say it wouldn't let you use up the data in the gigabag, have you installed data settings yet?
As you bought a gigabag, then to make calls etc. you would have needed airtime credit as gigabags include data only. You only need to buy a £10.00 goodybag if you want one.