Not necessarily like screenflow, but I have used Open Broadcaster ( http://obsproject.com/ ) for this purpose. All it does is recover various screen buffers to an mp4, but that is more than sufficient for this purpose in my opinion. Plus it is OSS.

Great for streaming video games to Twitch.tv or its variants, as well.

You could try LICEcap: http://www.cockos.com/licecap/, which is free for Windows and can produce an animated GIF. I haven't tried it for anything as long as 20 minutes, though.

Not sure what all the features of screenflow are, but Blueberry flashback has been pretty good.

http://www.bbsoftware.co.uk/