By default, Outlook displays the Play/Pause icon over the video thumbnail. Please check the below GIF behavior inside Outlook:
Animated GIFs are now supported in Outlook for Office 365 users only.
Animated GIFs will automatically play, looping three times, then pausing. A play icon will appear over the image once it's paused, allowing the recipient to replay the animation again.
Animation is turned on by default in your Outlook settings but can be switched off if required.