Google's documentation says:
Images will always appear in the Gmail app on your iPhone or iPad.
(This is different from opening Gmail in a browser on a computer, where I see settings that provide a choice: either "Always show images" or "Ask before showing images".)
How can I read Gmail messages on my iOS devices if I want to ensure that my opens can't be tracked?
P.S. The page linked above interestingly does not say:
"Sometimes, senders may know whether you've opened an email that has an image."
even though the "Computer" tab on that page does. I can't tell whether this implies that Gmail opens are NOT trackable in iOS.
Best Answer
As of Sep 3, 2019, version: 6.0.190811, you can Choose to be asked before external images are displayed automatically.
Gmail App - Account Settings - General