You can find it in Gimp 2.9.5 under
->colors->desaturate->color to gray
A bilateral gradient is a gradient that has one color in the middle and another on both sides. Select the gradient tool in Gimp. Once selected different options will appear, like linear, radial etc…and bilateral. Set the foreground color to neutral gray and the background color to black. Take a look at the “vignetting” layer in my image of the used layers