I solved it by myself. So here for the rest of the world or maybe for myself in a couple of months.

The following config is working:

{ module: 'MMM-BackgroundSlideshow', position: 'fullscreen_below', config: { imagePaths: ['modules/MMM-BackgroundSlideshow/BackgroundImages'], transitionImages: true, transitions: ['opacity'], randomizeImageOrder: true, slideshowSpeed: 500000, gradientDirection: 'both', gradient: [ "rgba(0, 0, 0, 0.00) 0%", "rgba(0, 0, 0, 0.00) 0%", "rgba(0, 0, 0, 0.00) 90%", "rgba(0, 0, 0, 0.75) 100%" ], //von oben nach unten, % = wo im Bildschirm startet es horizontalGradient: [ "rgba(0, 0, 0, 0.75) 0%", "rgba(0, 0, 0, 0) 15%", "rgba(0, 0, 0, 0.00) 85%", "rgba(0, 0, 0, 0.75) 100%" ] //von links nach rechts, % = wo im Bildschirm startet es } },

In addition, you have to change a line in the MMM-BackgroundSlideshow.js file in the getDom function (line 316)

OLD: this.createGradientDiv('right', this.config.gradient, wrapper); NEW: this.createGradientDiv('right', this.config.horizontalGradient, wrapper);

I will put this on GitHub as well.

UNSOLVED Google assistant background issue