How can I get the recipient’s Gmail iOS app to show my Google account’s avatar? When I added my avatar to:

https://aboutme.google.com/u/2/

it started showing up on mail.google.com. When I signed up for Google+, it started showing up on inbox.google.com. But in the Gmail iOS app, it shows up as a solid-color circle with the first letter of my email address.
Many sites have a delay when loading new profile images. Be sure to give it at least 48 hours to populate.