1. Gradient Image is the the Wipes transition section.
2. In Motion, put your gradient image into its own group. Use that group as an Image Mask to Transition A. To animate the gradient, Apply Color Correction: Levels filter to the group. Animate Black In and White In from 0 to 1 over the length of the transition (best to use Ramp behaviors so you can rig them later to allow for some smoothness - the ability to make the gradient edge smoother).
i.e. If you aren't confident in Motion yet, use Final Cut Pro's built-in transition.
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