I did some research and found that converting SWF banners to GIF animations is just a waste of time. Generated GIFs have way too high file size to be used on websites.
Why? Because GIFs are series of single pictures, while SWFs are algorithmic creations that uses only a few images.
I can give you an example.
This is my old work from times when I was coding in C++:
https://youtube.com/watch?v=VnhRtcIoiX0
This program contains only two bitmaps: a ball and a final picture pattern. Everything else is done by a simple algorithm. Generated size of EXE was about 1MB.
Imagine converting this thing to GIF. Instead of two bitmaps, it will have thousands of bitmaps. Imagine the size too.
That's why converting SWFs to GIFs is just waste of time.