Could you describe what this would look like and display?
The HR Gauge showed when doing a HR based structured workout
there is a HR gauge if you are doing a structured workout, saved through Garmin Connect and the target has been set as HR.
If you want to do a structured workout "manually" (no automatic laps and alerts when outside of zones), and only add an individual data field so that you can see your HR in a graph then do the following: go to Data Fields, choose layout and field as desired, then select Graphical > Heart Rate Graph