I have also noticed this issue on my Hp dv6t-7000 laptop with Intel HD 3000 graphics and a Nvidia GT 630m Optimus card. I eventually found a fix that worked from the Arch Linux wiki.

Basically my situation was that acpi_video0 was controlling the back-light, acpi_video0 is for ATI graphics. So, I added the following to the grub command line in /etc/default/grub acpi_osi="!Windows2012" acpi_backlight=vendor, so now it looks like this:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.semaphores=1 acpi_osi="!Windows2012"  acpi_backlight=vendor"

Then ran:

sudo update-grub2 

A reboot and the sensitivity is fixed.

Hope this helps.

Bind a keyboard shortcut

Gnome allows you to bind any key to run a command of your choice. If xbacklight worked, I'd suggest you use that. Since you said it does not, you can use the brightness script I'll include below.

1. Go to Settings → Keyboard → Shortcuts and scroll to the bottom of the list.
2. You'll find a + button. Click it to open the Add Custom Shortcut dialog:
   1. For the Name put "Brightness Up", for the Command, type brightness +10
   2. Click the "Set Shortcut" button and hit the key you want to turn the screen brighter.
3. Now do the same for "Brightness Down", using brightness -10.

b9's brightness script

Many external monitors cannot control the backlight lamp with xbacklight. But you can still control the brightness using software. Here's a script I wrote which does that.

#!/bin/bash
# brightness: Change all monitors brightness in software.
# by hackerb9, 2019

# Examples:  brightness 75;  brightness -5; brightness +10
# Usage:
#       brightess [n] [+n] [-n]
#       n       An integer from 0 to 100 specifies a brightness level.
#       +n      Increase brightness by n.
#       -n      Decrease brightness by n.
#               No argument shows current brightness level.

b=$(xrandr --current --verbose | grep Brightness)
b=${b#*: }                      # Remove "Brightness: "
b=${b#0.}                       # 0.30 --> 30
[[ $b == "1.0" ]] && b="100"
case $1 in
    +*|-*)
        b=$((b $1))             # b=b+10,  b=b-10
        ;;
    [0-9]*)
        b=$1                    # b=75
        ;;
    *)
        echo $b; exit
        ;;
esac

[[ $b -lt 0 ]] && b=0
[[ $b -gt 100 ]] && b=100

if [[ $b -eq 100 ]]; then
    b=1.0
else
    b=0.$b
fi

outputs=$(xrandr --current | awk '$2 == "connected" {print $1}')
for o in $outputs; do
    xrandr --output $o --brightness $b
done    

To install the script, paste it into a file called brightness, set it executable, and put it in your path. Or, you can cut-and-paste these commands:

1. wget https://github.com/hackerb9/brightness/raw/master/brightness
2. chmod 755 brightness
3. sudo mv brightness /usr/local/bin/

xbacklight

If xbacklight did work for you, you could use it as a drop-in replacement for brightness in the above instructions. (xbacklight +10 for Brightness Up, xbacklight -10 for Down.)