IntelliJ Idea Ctrl+Alt+Left shortcut doesn't work in Ubuntu

Ubuntu binds this key combination to Switch Workspace. Starting from Ubuntu 18.04, you need to use the command line to change the shortcut (source). To remove the offending shortcuts completely, allowing IntelliJ to use them, these commands worked for me:

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "[]"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "[]"

If you ever want to restore the shortcuts, use:

gsettings reset org.gnome.desktop.wm.keybindings switch-to-workspace-left
gsettings reset org.gnome.desktop.wm.keybindings switch-to-workspace-right

I just found out that my IDEA 2018.3 in Ubuntu 18.04 automatically uses default for GNOME keymap, and the Back/Forward shotcut turns out to be Alt + Shift + Left/Right, no need to change any system settings.

The right setting is combinations of already posted answers. To sum up:

1. Remove key bindings of Ctrl+Alt+Right/Left to GNOME enviroment

In terminal run:

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "[]"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "[]"

2. Assigne the bindings to Forward / Back actions in Idea IntelliJ

If you use Keymap setting based on GNOME keymap the defaults for Forward / Back actions are not Ctrl+Alt+Right/Left. It must be setted.

In Idea IntelliJ menu

File | Settings | Keymap 

search for

Main menu - Navigate - Forward

click on edit shortcuts and set it to Ctrl+Alt+Right

then search for

Main menu - Navigate - Back

click on edit shortcuts and set it to Ctrl+Alt+Left

Works on versions: Ubuntu 18.04.3 LTS Idea IntelliJ 2019.2.3