To start with the transparent background to make GIF/Video, tap 
icon.
You can add text, sticker, sketch to the transparent background.
Below is an example of GIF with Transparent background. ⬇️
Pro Tip!
Are you using the pro version of ImgPlay? You can also add your GIF sticker with transparent from your Photos Album! Check this out ➡️Customize your own Sticker 🎈