Open the Messages app
Select an existing conversation or start a new one
Tap the App Store icon next to the text field
Choose the #images app
Search for a GIF or browse the available GIFs
Tap the GIF you want to send
Tap the send button
If needed, press and hold a GIF to copy it, then paste it into iMessage
You can also use a third-party GIF keyboard or app and send from there