Additional chat-related features and issues within Microsoft Teams for business
***If the option to use a GIF isn't showing in the chat window, nor is there a "+" sign... this might help. It worked for me!
Open Teams
>Click the 3 DOTS (on the top right, next to your account icon)
>Click SETTINGS
>Click PRIVACY
>Toggle OPTIONAL CONNECTED EXPERIENCES to "on" mode.
Go back to your chat window and the GIFs option should show now.
Yes! It's this :)