I have heard that the dark gradient in PS happens because the color values are stored as their roots and later squared. The gradient function works with the root values instead of the actual ones, which causes the colors to become darker. I speculate that the gradient function of UE4 uses the regular values instead of the roots to interpolate the color.
It might be possible to simulate the PS gradient, if you take the roots of the individual channels, calculate their gradients, square the results and put them together again. (I am not at a pc with UE4 at the moment so I can not test if this answer will work or not).