Not really GS related, but from what i've done for the past year making banner, the easy/fastest way to do backup gif for me is :
(in a dual screen set up)
- create a doc in photoshop at the size of the banner you need (if i have 4 728x90 ads, all will be in the same doc.
- open all ads on the other screen
- in PS take a screengrab and past at each "frame" you need for the backup gifs (printScreen/Ctr+V)
in seconds you got all the layer, align them all together for the first ads, create the gifs frame, make a folder with the layer, duplicate and align the new folder to the other ads... (if the ads all share the same "timing" you can do this really fast.)
One time i had to make all the gifs for a campaign with more than 300 variations on the ads, it took me less than an hour with this method.