[Coder-san]

In Photoshop, go to Menubar ->Windows -> Animation for displaying the Animation (Timeline) or Animation (Frames).
You can work in both of them. Timeline is like what Video-editors provide, whereas Frames is the basic menu for Animation.
Older versions only have Animation in Frames menu.

Both are pretty easy to work with. From which Timeline is a bit confusing at first, but has more options.

In this particular GIF, you have to Rasterize the Text Layer first, then keep copying the layer and adding Gaussian Blur of about 2 pixels.
Then after it's all blurry, copy the old frames back-to-back showing reverse animation of the frames so far.

That's it.