Doubt that would be possible the way things have been set up
All the major companies work the same way if you buy a package, but gg's payg rates are very competitive sa less of a problem with gg.
If you want a further allowance, simply have a spare sim and load a gigabag onto that. It will never be cost effective enough to have small bundles of data to add on to an existing bag.
So the choice is to correctly predict usage and buy accordingly from the current three gigabags, or hedge your bets, get what you think might be enough, and then if you run out, just operate a second account with another bag that would work out cheaper than fulfiling the remainder of your month's usage at 2p per MB
Remember, Gigabags already have a buffer for when you go over limit before being charged, and then it's only 2p per MB thereafter. Buying another gigabag in the same month could work out more expensive
well you would of think so , i think this posting is a very good one , well done