The following commands must be run in the user shell.
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-left
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-right
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "[]"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "[]"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-left "['<Control><Alt>Left']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-right "['<Control><Alt>Right']"