There's a password on my Google account, I totally expect to have privacy for anything I didn't choose to share with other people.
The hash is kind of metadata recorded by Google, I feel like Google using it to keep child porn off their systems should be reasonable. Same ballpark as limiting my storage to 1GB based on file sizes. Sharing metadata without a warrant is a different question though.
As should be expected from the lawyer world, it seems like whether you have an expectation of privacy using gmail comes down to very technical word choices in the ToS, which of course neither this guy nor anyone else has ever read. Specifically, it may be legally relevant to your expectation of privacy whether Google says they "may" or "will" scan for this stuff.
The hash is kind of metadata recorded by Google, I feel like Google using it to keep child porn off their systems should be reasonable. Same ballpark as limiting my storage to 1GB based on file sizes. Sharing metadata without a warrant is a different question though.