Double-check that your iPhone is connected to mobile data or a Wi-Fi network and has a proper internet connection. Last but not least, GIFs not working on your iPhone could be related to a more general issue. Open the App Store, tap the profile icon in the top right corner, swipe down on the page to check for new app updates, and see if there's an update available for your keyboard. If you're using a third-party keyboard and having GIF issues with it, it could be running outdated software. Find the Messages app, swipe up to close it, and then open it again. Swipe up and hold from the bottom of your iPhone's home screen to open the Recents menu (or double-press the Home button if your iPhone has one). If GIFs continue to be broken on your iPhone, it may just be that the Messages app needs to be restarted.