We've successfully tested our timer in all major email clients. The only client that does not support animated GIFs and therefore our timer are the newer versions of Outlook (2007, 2010, 2013, and 2016+)
For more details, please see this helpful write-up by Litmus: https://litmus.com/blog/a-guide-to-animated-gifs-in-email
Although the timer does not animate, it will show the first frame providing the recipient with a decent fallback experience. You can also read the support document from Microsoft here: The animated graphic in my e-mail message doesn't work.