Ohhh! Apparently they added actual grayscale to Luban if you don’t pick a dither option! Nice find, but very tricky to dial in and is only really good on materials that respond to and darken at different levels of laser power.
What it does it convert the image into a heatmap and changes the laser power depending on darkness, and doesn’t seem to have a way to control the range. In Lightburn this is set with Min/Max (max power is for purest black, min is for purest white) and modulates % between them. Unless you want to waste a lot of material dialing in the settings, make sure to use a dithering solution. I suggest Stucki for real images and Atkinson for high contrast/flat colour like cartoons or sketches. 
If you select None it’ll process in grayscale mode with varying power, any of the other options will dither the image, and use your set power.
Example snippets;
Using None and it varies the laser power;
G1 X174.60 S65.132
G1 X174.80 S70.728
G1 X175.00 S75.584
G1 X175.20 S80.114
G1 X175.40 S82.376
G1 X175.60 S82.557
G1 X175.80 S84.139
G1 X176.00 S89.652
G1 X176.20 S96.654
G1 X176.40 S100.794
G1 X176.60 S103.111
G1 X176.80 S108.771
G1 X177.00 S113.361
G1 X177.20 S119.351
G1 X177.40 S124.847
G1 X177.60 S128.691
G1 X177.80 S131.508
G1 X178.00 S134.191
G1 X178.20 S135.837
G1 X178.40 S139.047
G1 X178.60 S141.644
G1 X178.80 S142.366
G1 X179.00 S143.39
You can see the power dictated by the S# parameter changing with every line, and even very low (I had power set to 100, which is S255). If your material only responds to like, 90% or higher power at your given speed, then none of these lines will create a gradient burn.
Selecting any other option will dither the image, making it dots, as such the laser will simply turn off and on at your given power.
G1 X72.20 S0
G1 X72.60 S255
G1 X72.80 S0
G1 X73.60 S255
G1 X73.80 S0
G1 X74.60 S255
G1 X74.80 S0
G1 X75.80 S255
G1 X76.00 S0
G1 X76.60 S255
G1 X76.80 S0
G1 X77.20 S255
G1 X77.40 S0
G1 X78.40 S255
G1 X78.60 S0
G1 X79.40 S255
You can see the difference in the preview as well, dithered will be broken into dots and lines;
And actual grayscale will be smooth gradients changing power;
Grayscale when dialed in just right can achieve some amazing results, but it’s so time consuming and the organic nature of things like wood throw it off so much.
In Luban 4.9.1 try setting the algorithm as Stucki and use a Line filled engrave.