install gnome-tweaks
jacek@comp:~$ gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-5 ‘["<Alt>5"]‘
jacek@comp:~$ gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-6 ‘["<Alt>6"]‘
jacek@comp:~$ gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-7 ‘["<Alt>7"]‘
jacek@comp:~$ gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-8 ‘["<Alt>8"]‘
jacek@comp:~$ gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-9 ‘["<Alt>9"]‘