While the GIF plays for the first time, you can click , to stop it immediately.
After the GIF finishes looping the first time, just hover over it and click to play it again.
Webex | Stop and Replay Animated GIFs
When someone shares a GIF in a space in Webex, the GIFs play automatically but stop looping after a few seconds. But, you can play them again whenever you'd like.