How to disable mouse acceleration in Ubuntu 20.04, 19.10 or 18.04 (Gnome)

Mouse acceleration dynamically adjusts the sensitivity of the mouse cursor based on how fast you move the mouse on the mouse pad.
Although mouse acceleration may be beneficial in some cases, it may damage your goal in FPS games. This is because the movement of the mouse depends not only on the movement of the hand, but also on the speed, so any consistent behavior cannot be achieved, thereby reducing accuracy. This is why it is recommended to disable mouse acceleration when playing first-person shooter games.
In desktop environments such as Xfce, KDE Plasma and Cinnamon, you can find the option to disable mouse acceleration in the “Mouse” section of “System Settings”. However, you cannot find any mouse acceleration options in Gnome’s system settings. Recent Gnome versions (3.22 and later) all have a “hide” mouse acceleration setting. By installing Gnome Tweaks (Gnome Tweak tool) or Dconf editor, you can change the mouse acceleration configuration file to “flat” to disable mouse acceleration. GnomeTweaks can also be used to disable mouse acceleration on Budgie Desktop.
Ubuntu and Debian users can install Gnome Tweaks using the following command:

sudo apt install gnome-tweaks

To disable mouse acceleration, start the Tweaks application and click Keyboard & Mouse On the left and select Flat Down Acceleration Profile For your mouse:

Since this feature requires Gnome 3.22, it exists in Ubuntu 20.04, 19.10, 19.04, 18.10, and 18.04, but it does not apply to Ubuntu 16.04 or earlier. You will also find this feature in Fedora 32, 31 and 30, Debian Stretch and later, openSUSE Leap 15.0, Arch Linux, etc.
For a general method for disabling mouse acceleration on Linux, you can check Arch Linux Wiki.