Your GIF can be found in the Camera Roll or All Photos section of the Photos app. You'll notice that it doesn't animate when viewed in the Photos app. The compose message or email screen will appear with your GIF. If you just want to view the GIF yourself, send the email message to your own address.    Once the message is sent, you'll see your animated GIF in the conversation list.

TL;DR in Vietnamese:
Mở Photos (ảnh). Chạm để mở ảnh GIF. Lựa chọn người nhận. Gửi tin nhắn.