Hi ,As your phone is an android based phone you maybe able to use an app from the play store to install the giffgaff APN settings ,see the link below
Or you could install an android giffgaff app
If that doesn't work on your phone then
you could look to see the internet settings (giffgaff APN) for most phone's here and add them manually to your phone http://bit.ly/ggjellybean
You'll need to add some settings (called an APN) manually for your phone to access the internet and send MMS messages on giffgaff. Here's details of how to do that for most phones -
Android - http://goo.gl/iEsCXZ
iPhone - http://goo.gl/cPcjgj
Blackberry - http://goo.gl/Nnr5KX
Nokia Lumia - http://goo.gl/yInbYG
Windows - http://goo.gl/l5GSJS
Other phones - http://goo.gl/t3UbYT
You may need to turn your phone off and back on at this point.
If you're still having problems, there may be a problem with something other than your phone's settings. Firstly, check the obvious stuff. Make sure your phone is unlocked or locked to the O2 network or your giffgaff sim won't work. Check to see you have an active goodybag with available data or a PAYG credit balance on your 'My giffgaff' page. Next, check if your phone has been switched to airplane/flight mode as this disables all connections, including mobile data. Lastly, check you have mobile data enabled. If you're still having no luck, check for signal coverage and maintenance in your area using this site -
Finally, you can try what's called a manual roam. Check out this site for info and instructions -
PLEASE NOTE - Apparently the 2020 text settings are incorrect at present as is the giffgaff APN app, so I'd recommend not using those for now. Doing so can redirect you to the wrong site when you want to top up.
Hope that helps