How can I easily switch audio output in Ubuntu 17.10 and later with GNOME?

You may use the Sound Input & Output Device Chooser GNOME shell extension.

(The extension) Shows a list of sound output and input devices (similar to gnome sound settings) in the status menu below the volume slider. Various active ports like HDMI , Speakers etc. of the same device are also displayed for selection.

(screenshot source: extension's homepage at extensions.gnome.org)

You may visit its GitHub page for more info.

Refer to this to learn more about installing and managing GNOME Shell extensions: How do I install and manage GNOME Shell extensions?

You can use the Sound Switcher Indicator from Dmitry Kann's PPA

sudo add-apt-repository ppa:yktooo/ppa
sudo apt-get update
sudo apt-get install indicator-sound-switcher

From the OMG Ubuntu article:

Sound Switcher axes the need to use System Settings and, in just two clicks, lets me hop between audio input and output devices straight from the system tray.

An older tool, but coming with more options than indicator-sound-switcher (and without the need for a new PPA), is pasystray.

sudo apt install pasystray

It should be launched as PulseAudio System Tray.

You could add pasystray to startup programs list.

The output switch is under "Playback Stream" section.

Beside the fact it doesn't need a new PPA it also has a lot of supplementary options.