giffgaff recently amended the various goodybag, goodybag with BIS and gigabag product offerings.
The £12.50 (3GB) gigabag has been withdrawn.
Upon arriving at the gigabags page with the intention of buying the old £12.50 gigabag again and seeing only the £7.50 (500MB) and £10.00 (1GB) options listed, my first thought would be to leave giffgaff and swap to another network.
In order to retain customers, this page should have a short piece of text explaining the £12.50 (3GB) gigabag has been withdrawn and direct members to look at the £12 goodybag product. This has the same amount of data for 50p per month less cost.
This is basic marketing practice. It's easier to retain existing customers than to find new ones.
Edit: As suggested below, it could also mention the 5GB for £15 goodybag there as well.