If you use the iCloud Photo Library, can be a problem.
When using “Optimize iPhone Storage” iCloud Photo Library, it automatically deletes photos saved in the device and saves them into iCloud. Therefore GIF files cannot be retrieved from iCloud. GIF Viewer app helps you perform on the GIF files saved within the device only. (For iOS9 and higher, GIFs uploaded to iCloud can import normally.)
Choose one of these two methods and practice it.
Method 1. “Download and Keep Originals” under the setting of iCloud Photo Library
Settings App -> Photos & Camera > iCloud Photo Library, select “Download and Keep Originals”
* Files already saved in iCloud can be retrieved again into your device later.
Method 2. Copy GIF into the album within GIF Viewer app
GIF files copied into the album within GIF Viewer app do not get affected by iCloud. If you would like to use “Optimize iPhone Storage” iCloud Photo Library, copy the GIF files into the album of GIF Viewer App before uploading them into iCloud.