Now adding GIF in story is becoming more popular and trending day by day.
people use different GIF according to story and mood.
open your Instagram home page and click on top left icon like this
to post different GIFs on Instagram story. click on this option like this
now a new options bar will open up on top right corner.
click on icon like this
now you will find option of GIF as seen below... click on GIF icon to use different GIFs
you can even search for particular GIF by typing specific keywords on search bar.