The gmail product forums, monitoring sites like reddit, HN, and Twitter, focus groups and UX studies, internal dogfooders and trusted external testers, metrics from experimental or holdback groups, etc.
I sincerely hope they do that. Given the evidence though, I also doubt it, or at least I feel the feedback is quickly dismissed under the "every UI change breeds discontent" rule of thumb.
The gmail product forums, monitoring sites like reddit, HN, and Twitter, focus groups and UX studies, internal dogfooders and trusted external testers, metrics from experimental or holdback groups, etc.