Was leptonica built without support for gif? Homebrew doesn't seem to include support for gif:
$ brew info leptonica
leptonica: stable 1.69
http://www.leptonica.org/
Depends on: libpng, jpeg, libtiff
/usr/local/Cellar/leptonica/1.69 (240 files, 7.5M) *
https://github.com/mxcl/homebrew/commits/master/Library/Formula/leptonica.rb
==> Options
--with-libtiff
Build with libtiff support
--without-jpeg
Build without jpeg support
--without-libpng
Build without libpng support
You could try building leptonica from source, or just convert images to some other format first:
convert file.gif temp.png; tesseract temp.png output