You don't need to analyze content on the Gmail server to get content analysis to the Gmail server (if that is necessary at all). If Google's primary intent is

You don't need to analyze content on the Gmail server to get content analysis to the Gmail server (if that is necessary at all). If Google's primary intent is to make sure that users keep their email encrypted completely, smart clients (like browsers and mobile apps) can do what Gmail servers can't. Look at what the Google-provided Gmail Offline extension does. Now remember that End-to-End is just an extension, like Gmail Offline, except that this one also decrypts email on the client. Once email is decrypted on the client, analysis can be performed (like indexing the email for search), and that analysis can be sent to the Gmail servers or used to perform ad calls directly (I'm not saying that Google is doing this, but technologically all of this is pretty easy given everything else already done).