At the moment, it is not possible to export a GIF or video with a transparent background. This is a feature we will add in the near future.
In the meantime, here are 2 workarounds:
Solution 1: Use the “Screen” Blend Mode
Use a black background
Export the video
Import it in a video editing tool like Final Cut Pro
Layer the text effect video on top of your other video or image
Set the "Compositing Blend Mode” of the text effect video to “Screen”
Solution 2: Use the Chroma Key tool
Use a green background
Export the video
Import it in a video editing tool like Premiere Pro
Layer the text effect video on top of your other video or image
Use the Chroma Key tool to remove the green pixels of the video