When I was researching this product area, I found a bunch of similar tools that summarized on the inbound side. I thought it would be more interesting to experiment on the outbound side as the main initial focus, which I haven't seen.
I do have a generic popup that you can to summarize any passage, so you could copy-paste some long passage (email, article) to piggy back on the functionality. But it isn't tightly integrated into the Gmail UI as of now.
I do have a generic popup that you can to summarize any passage, so you could copy-paste some long passage (email, article) to piggy back on the functionality. But it isn't tightly integrated into the Gmail UI as of now.