Open the GIF in Safari or Chrome
Use a browser that supports animated GIF playback
Make sure the GIF is not opened in Preview
Save the file with a .gif extension
Disable any image optimization that may convert it to a static image
If using Messages or Mail, send the GIF as a file attachment
If using Finder Quick Look, open the GIF with Spacebar
If the GIF still does not animate, try opening it in a web browser
If needed, convert the GIF again using a tool that preserves animation