First launch QJackCtl and configure the JACK server. Do not start the server yet. Click Setup... button.
From Settings tab select Advanced tab.
Select as an Input Device and choose the hw:CODEC USB Audio CODEC.
Launch the Ardour and select the following settings for the new session and click Start button.
Once the Ardour is started check if the JACK is running and you see the connections. If JACK is not running click the Start button.
Click the Connect button and then Expand All. You may need to click Refresh too.
Add a new track(s).
Right click on the track and select Inputs... and Outputs... to configure the connections respectively.
Repeat the same for all other tracks you added and the Master track.
You can also configure Inputs... and Outputs... in the Mixer.
Click the Mixer button in Ardour main window on the toolbar.
Click the "Track Name" in the Mixer for each track and select Inputs... and Outputs... to configure them.
You can start recording now.
Note: Of course, it is a very basic configuration to just get you started. I guess, it should work with any USB audio interface really. If you want to learn more please see the reference below.
Ardour Manual: http://manual.ardour.org/toc/
Zoom 2g.1u: https://www.zoom.co.jp/products/guitar-bass-effects/guitar/g21u-guitar-effects-pedal#overview