From the "Get 1GB extra data on us" pop-up
"It’s never fun running out of data, which is why we’re giving members a little extra.
Once you’ve bought your 3rd goodybag, we’ll boost your data by 1 GB whenever you buy a £10, £12, £15 or £20 goodybag._"
It's been that way for sometime now.
You can go PAYGO or choose a different goodybag every month if you wish or even leave and return at anytime as there is no contract. giffgaff use the term plan here so that people coming from other networks can relate to the terminology.