Please try texting the word "settings" to 2020 this currently works for some devices,but not all unfortunately. If texting the word "settings" to 2020 doesn't work for you,you may have to manually enter your giffgaff "settings"
If you would like to take a look at a fantastic guide to setting up your device for both MMS and Internet Data,please click onto the link below,cheers.
Depending on what phone you have there are 3 ways to do this. 1 the easy way do what the others have said and text settings blar, blar. 2 slightly inconvenient but it might work is to find the giff gaff APN app, there is also another app relating to giff gaff. Or 3 this is will test your patience to the limit and get your fingers at the ready, it is to manually type everything in as most likely mentioned above. thank you and good luck.