Running Pace Range Gauge Data Field

Not available outside a planned training. 

For free runs, how would the watch know what is the "green zone"? Depends whether you are doing a base run, tempo run, etc. At minimum, you could create workouts of your own with one step with a suitable pace range and no fixed duration ("until lap press"). Then you could start one of those to get the gauge.

You'll have to create a workout with a target pace so the watch can determine what is green or red.

You can setup pace alerts let you know if you are within or outside your desired pace range.  Not the same as a graphic, but does give you the information you want. 

you can try this data field, which will give a graphic and easy to look level of pace.
Pace Zone Data Field | Garmin Connect IQ

What I'd love to know is how you have your pace range guage so large? I have a 7s, and instead of a chunky bar with a thick arrow, I have a really narrow one with just a thin line which is REALLY hard to read mid-run.

Is the above from a "Garmin Coach" plan? Wondering if that's the different

I have a 7 Pro and I get the same single pixel line to indicate the pace - no chance of seeing it without my reading glasses!

Create a workout with a target pace range, then automatically when you do this workout the watch adds a data screen to the usual running app usually located at the "bottom" of the data screens. As you start the run, scroll down and you go to that data screen for your run. If not, then the watch will bring it up briefly whenever you are too slow or too fast of your target pace range. This data screen is only available when you specify a pace range not just a pace target. For a target pace you can only add the "running mate" data screen where you have a virtual partner running with you at the target pace. All workout tools with target paces and distance are available once you select the running app and then "up" key (fenix 7-8 all iterations). Now when you do a "suggested workout", usually "threshold" when you start the run the watch put that data screen on top, this is why you see it. So far, there is no way to modify the way data screens appear in workouts, or better said, I have not figured out any way. 

I wish you find this useful