jasc paint shop pro has an animation tool, even the “microsoft gimp animator” can be used to do what you want… or even the gimp [if you’re a masochist… or have found a better way to load images as layers]
[select the last file, then hold shift and press home to select the rest of the files]
If you want to use Free Software, here’s a fast tut:
You must save your Blender animation in Jpg or PNG image format( preferred)
Then in Gimp create an image with the same dimensions of your animation, ex: 320x240
With a file browser select all the images in the animation and drag them over the new image window
This will create all the layers for you ( what a chance)
Then in the Filters/Animation menu choose Optimize for GIF
Then save the file in Gif format
Choose Animation in the box dialog and standard indexed conversion
That’s all
This is the basic procedure
Bye and good luck