Hi there. Apple now recognises giffgaff as a carrier so it's now easy to set up your internet/mms.
Assuming you are currently up to date with the latest iOS 7.1.1 and have access to the internet with data allowance/wifi, go to SETTINGS > GENERAL > ABOUT you should be offered an option to update your carrier settings.
Or plug in your iDevice to your computer to connect to iTunes (make sure iTunes is up to date) to install the carrier profile.
If your iPhone is running an older version, you will have to update it to the latest version to install the carrier settings.