Reasons to include it in the repository:
The gnome-gmail package allows desktop mail actions, such as "Send File as Email" or web 'mailto' links, to be handled by the Gmail web client. It responds to the preference of many users to work with Gmail directly on the web rather than importing/syncronizing their account to a dedicated desktop email client, adding the functionalities mentioned above.
Is it open source?:
Number of users anticipated
Hard for me to say. The package is standard in Debian and all its derivatives' repositories, so a few millions may be using it.
Link to the source repository: