There is an error in the documentation file doc/gif_lib.xml, which is the source for gif_lib.html.
The error is in the documentation of the functions EGifPutExtensionLeader
and EGifPutExtensionBlock, which changed from previous versions of giflib, which used EGifPutExtensionFirst and EGifPutExtensionLast. The parameters to these functions in the documentation don't match the parameters to the functions in the giflib/include/gif_lib.h file that is installed (which is ground truth), and need to be updated.
This is an important point, because folks that (like me) have to update source code written for older versions of giflib to be compatible with the latest release are going to be very confused when trying to figure things out.
If I felt more confident in my knowledge of giflib I'd submit suggested text for the corrected documentation, but I'm still feeling my way around. I'm 100% confident, though that what is there is NOT correct.