I had it for awhile and then one day it was gone! Good to know I'm not alone - I have a Samsung s4 and update my phone all the time. Still no answer as to what is going on?
If there's still people having troubles on this subject.. try tapping on the smiley face icon beside the comment bar and gif's will come up as an option.
(I have an s8+ and this works on mine)
If your gif button is missing on Facebook go to your app store and search GIF keyboard tenor and install and then your gift button will reappear for Facebook