If you can load the file into Strava or Golden Cheetah it's fairy straightforward to select a 20min segment. Worst case you could export to a csv file and use Excel.

Go to Garmin Connect, view the activity details.
Find the area you care about and drag to view stats. Once you release your drag on any area below the activity view will crop and show you stats for only the snippet you highlighted.


You could also pick from the 'Max Average' dropdown and pick the best 20 minutes to see your effort exactly snipped to 20 minutes.

Even more specifically, when you click on a cycling activity, it will default to the 'stats' screen. Then, scroll down to the power information and just click on the drop down for "Max Average" which is along the power data. You don't have to search out for the best, Garmin will and auto-populates it for you along with corresponding HR/speed/elevation or whatever your custom charts are.

This is done when you click on any one of the drop downs, including 20 min average *(the others are: 5sec, 1min, 5min 20min, custom), the data automatically populates to that segment with the 'best average' for that ride. The average for that chosen interval is noted right next to the drop down, followed by the time stamp -so it might looks like:
219w (0:05-20:04)

Then click on the double sided arrow in the upper right box and it will expand it to the entire screen. You can add overlays to that screen also.

*(But, TBH, I use GC too...as my main source of power data processing)

Thanks,
I'll try it again. I was doing essentially what you are showing, but it didn't seem to work. Maybe I was doing something wrong.
Appreciate the graphic.

Thanks-Appreciate the response.