How do I log a defect against Google's gmail app?
I don't believe Google has a public bug tracker. However, there is an unofficial one available at code.google.com/p/googlebugs. I did some extra looking, and there doesn't seem to be anything like Apple's own bug reporter or the community-driven one. 

Thankfully, Gmail.app has a built-in mechanism to send feedback with. Starting with your inbox view, open the menu by tapping on the menu icon in the top left. Then: 

Tap the gear icon &gt; "Help and Feedback..." (see picture) &gt; "Send Feedback"

This will provide you with a form to send feedback to Google.