His video-tutorial is based on an old 90ies Photoshop effect book, called the 'WOW Book 4'.
Because Gimp doesnt have the same filters as PS, i recreated the effect by using the Displacement Filter.
Instead of using the channel technique that didnt work for me, i just made a frame by shrinking the alpha selected text by an amount i wanted for the bevelling, then cut and blurring the frame.
So i end up with a blurred frame on a transparent layer and used that as my Displacement-Map.
You chose pretty big values for the displacement because you want a zigzag effect on the original image.
And thats really it.
I added a bevel with layerfx and applied anisotropic smoothing in G'MIC to make some of the jaggy distortions a bit nicer.
As you may remember, i have talked about landscapes as gradients inside a texteffect before. In case you missed it, here are the relevant posts: