goodybags are 30 days (one calendar month) contracts, they are just not credit agreements.
Doing credit accounts introduces a whole new tranche of legislation and regualation to comply with, and will make the "giffgaff stole my money" reports even more common. (Actually my impression is that a surprising number of giffgaff users are so close to the edge financially, that they cannot afford even the odd £5 or £10 over their budget.)
If you need a contract there is a provider oput there giving much more data coverage and speed but you will pay much more and be tied in for 24 months paying and paying. The advantage of giffgaff/pay as you go, no contract no credit check can stop anytime without extra cost.