Do we really need a separate app for this?

In the beginning

When we first released Gmail Notes in early 2009, there was no good way to attach notes to Gmail conversations. FYI, there were a few not-so-good, homebrew approaches:

What about Google?

More recently, there was some hope that Google Keep would be expanded to work with Gmail. But so far, there's been no hint of that from Google.

Notes for GMail

One serious alternative to Gmail Notes became available in mid-2012. Called Notes for Gmail, this apps makes some improvements over Gmail Notes, including:

As for downsides:

Also, Notes for GMail has called itself beta since it was introduced in 2012, and its Chrome extension is not available in the Chrome web store. That makes installing a manual process, but it is still straightforward.