It depends on the gradient that is used. You could use UV or worldposition or even a hand painted gradient texture. You could hand paint a gradient that reveals however you want. black areas would dissolve first, white last.
A very convenient function here is BoundingBoxBased_0-1_UVW. It gives you the local world space. Use w ( aka b aka z) for vertical, or x,y for horizontal options. You can also rotate the results to get a non cardinal axis for blending.
Then blend that with a bit of a noise texture if you don’t go the wholly hand painted route.
To flip the direction. You could use one minus to invert or multiply by -1 and use a manual offset to control the cutoff points. Things like the glowing edge are just creating another gradient with a longer blend.