Giphy for Messenger is a free app which helps you find and use animated GIFs inside Facebook Messenger.
Call up the app and you can view trending categories, maybe giving you an idea what to send.
Alternatively, Giphy for Messenger comes with a long list of possible reactions for any conversation: "Applause", "Awesome", "Eye Roll", "Face Palm", "Goodbye" and many more. Tapping the one you need displays a range of GIFs on that theme and you just choose your favourite.
If that doesn't work, there's also a search box where you can enter whatever keywords you like.
What's New in Version 2.2.2
+ Misc bug fixes
+ Improvements around Partner Channels
Its integration needs work, but if you're a fan of animated GIFs then Giphy for Messenger is a fun way to find and use even more