If you get a warning, try the unencrypted download.
Here is a plugin to open and save animated GIFs/PNGs, one frame into each layer.
You will have to rename the files to .agif or .apng to be imported and rename them after saving.
[ I did this to avoid conflict with the built-in file handlers as animated GIFs have the same extension as normal GIFs. Paint.net can already open normal GIFs, so we can't use the standard GIF extension as paint.net's native hand