This article explains how to copy and paste a Dubb video and animated GIF on mobile to Gmail.
Select the video that you want to attach in your email, and then tap the Copy button.
Once redirected to the Dubb page on your browser, click the Copy for Email button. This will copy the animated GIF and link.
Then go to your Gmail app, compose an email or reply to an email, and Paste
And it should show the animated GIF Preview of your email, and you may send it.