user3907: I want to add a gradient transparency mask so that the front layer will slowly transform into the layer below. Can anybody suggest something?
I'm not talking about transition in time, but transition between layers. This is an example of video I want to get:

I think that in old final cut you could do this with black to white gradient and filters.

user3592: If you're talking a special, unique transition that you can't find in the effect browser, you have to use Motion to create the transition yourself, and publish it to FCPX.  If you're just looking for a slow dissolve, there's already a dissolve transition available.  Just drag it from the effects browser to your edit.
Edit: Here's one I created in Motion.  Delete the .xml extension that google adds to the filename; it should be called "SideBySideGradient.motr", not "SideBySideGradient.motr.xml"  Put this file at ~/Movies/Motion Templates.  If you don't already have a Motion Templates folder in your [home directory]/Movies, create one there, drop this in, and look for "SideBySideGradient" in your FCPX effects browser under the transitions category.  Drag this transition between two clips on your timeline, and VIOLA!
I realize that this is a pretty simple application which might not suit your needs exactly, which is why I encourage you to use motion and try it for yourself.  It's not hard!  If you need help, feel free to ask.

user10589: I don't know if you ever found a solution, but FCPX does have a Graduated Mask that works.
To use it:

Position clip1 over clip2 on the timeline.
Add the "Graduate Mask" filter to clip1.
Animate both "Center" and "Target" properties of the effect to move left to right.


3907: <p>I want to add a gradient transparency mask so that the front layer will slowly transform into the layer below. Can anybody suggest something?</p>

<p>I'm not talking about transition in time, but transition between layers. This is an example of video I want to get:
<img src="https://i.stack.imgur.com/t7hES.jpg" alt="enter image description here"></p>

<p>I think that in old final cut you could do this with black to white gradient and filters.</p>
 3592: <p>If you're talking a special, unique transition that you can't find in the effect browser, you have to use Motion to create the transition yourself, and publish it to FCPX.  If you're just looking for a slow dissolve, there's already a dissolve transition available.  Just drag it from the effects browser to your edit.</p>

<p>Edit: <a href="https://docs.google.com/file/d/0B6aKxDC1rLtqbGxnZ2Z5WGZqb0U/edit?usp=sharing" rel="nofollow">Here's</a> one I created in Motion.  Delete the .xml extension that google adds to the filename; it should be called "SideBySideGradient.motr", not "SideBySideGradient.motr.xml"  Put this file at ~/Movies/Motion Templates.  If you don't already have a Motion Templates folder in your [home directory]/Movies, create one there, drop this in, and look for "SideBySideGradient" in your FCPX effects browser under the transitions category.  Drag this transition between two clips on your timeline, and VIOLA!</p>

<p>I realize that this is a pretty simple application which might not suit your needs exactly, which is why I encourage you to use motion and try it for yourself.  It's not hard!  If you need help, feel free to ask.</p>
 10589: <p>I don't know if you ever found a solution, but FCPX does have a Graduated Mask that works.</p>
<p>To use it:</p>
<ol>
<li>Position clip1 over clip2 on the timeline.</li>
<li>Add the &quot;Graduate Mask&quot; filter to clip1.</li>
<li>Animate both &quot;Center&quot; and &quot;Target&quot; properties of the effect to move left to right.</li>
</ol>
