Can I show a gif animation in a Photos.app slideshow?
Unfortunately the stock Photos app simply does not support the *.gif file type. To view *.gifs on a Mac you have 3 options:


Locate the file in a Finder window, highlight it, and then press the
spacebar
Locate the file in a Finder window, right-click, and choose to open
in Safari
Install a 3rd party application with *.gif support (such as 'PhotoScape
X' or 'Fragment', both free in the App Store. Unfortunately neither of these have the ability to create slide-shows)


I understand none of the above options help you with your slideshow project, and for that I apologize, but there is currently no way to make this happen.

With the addition of the Photos.app extensions (as of El Capitan) I was expecting someone to come out with an extension for *.gif support, but I have yet to find one or hear of one after searching tirelessly through the App Store and performing countless Google searches.