Gmail Doesn't Respect Dark Mode System Preference on iOS 13
UPDATE:

Was surprised to see Dark Mode magically working on my iPhone's gMail. I had not fiddled with anything lately, but voila, there it was. However, when I checked my iPads, these did NOT have Dark Mode enabled. Checked the versions on the iPads and these had the same one as the iPhone: v.6.0.200412.  So what was different?  I rebooted the iPhone recently, but NOT my iPads.

Upon reboot of the iPads, Dark Mode was now working on gMail on all my devices.

How to get Dark Mode working: (3) Steps


Set Dark Mode as the system-wide preference in: "System Preferences" > "Display &amp; Brightness" > "Appearance" > "Dark"
As @Lulucmy rightly noted in his answer, Google must enable the feature on their side or Dark Mode ain't happening for you.
Reboot the device periodically: Google will not notify you they are enabling Dark Mode. I had periodically closed gMail from time to time since I last updated it 01 May, and Dark Mode was still not working in gMail.  Only when I rebooted my iPhone did the change seem to be effective. 


So Dark Mode will respect the global System Preference, but only where Google has enabled it on their side. So I guess patience is also required for those eager to extend their battery life using gMail in Dark Mode.

Anyhoo, my observations on how my own Dark Mode Hell was resolved.  Hope this saves others cycles trying to resolve the issue-