How to make a gradiated lower third in Photoshop with an alpha channel?

A little late to the party, but...

When you finish with your design if you want to keep the file simple and not use a .psd, use the File > Quick Export as PNG to preserve the transparency.  In designing, if you use the gradient editor, you can adjust opacity values so that the color on one end goes transparent.  Use the midpoint adjustment marker to tweak how fast or slow your switch to transparency takes place.  Masking certainly works too, but sometimes its quick and easy to just apply a gradient.  In the gradient editor presets, you'll find a black to transparency option that you can adjust the black color and add additional colors if you prefer.

JuliaS2000

Julia, a wee addendum, Photoshop has useful range of presets for video formats complete with Guides for safe area and title safe area.

If you scroll down there are a range of presets with things like lower thirds, but these have to be downloaded.  They are free though.

Premiere pro has similar functionality.  The lads in the Premiere Pro forum would know how this stuff works, even if you are not using Prem Pro.

What app are you using the lower third in?  Premiere Pro or Photoshop maybe?  Prem Pro works really well with PSD files.

Anyway, create a document the same size as the video frame

Create a new layer and add a layer mask

Select the lower third  and the gradient tool with Foreground to transparent

With black as the foreground colour and the layer mask selected, run your gradient from right to left

The alpha channel will only show in the channels panel while the layer is selected, but you could drag it to the new channel icon to make it permanent.

Design the bar. Use a layer mask of white (show) to black (hide) to fade out the layer. Select the mask and save the selection as a channel to create the alpha channel.

You'll have to save to a format that supports alpha channels. PSD and TIFF are good choices. Don't use JPEG.