Most of the examples of Google closed apps that are not part of the AOSP release are in fact apps that are based off of Google data-center services. Would it really help Samsung if the source to the Gmail app was open? Since Google controls the server side, and the client-server protocol, it limits the amount of innovation they can do. The web equivalent would be like claiming that Chrome OS isn't open because the so…
You're partly right, but in regards to the Gmail app, what I don't like about Android is that it doesn't come with a good email client that's not GMail. Android's GMail client is very polished and there's not much in it that's specific to Gmail, it could be a generic email client that works with POP3/IMAP/SMTP and other standards as well. The experience of users that aren't into Android's own services do suffer. But…
GMail uses a proprietary protocol that is decidedly not imap. It has features like colorized labels and inbox prioritizing that just isn't present in imap. Furthermore it has completely different performance characteristics (e.g it is much faster) than what imap email can provide. No one seem to have any details on exactly what the protocol is or how it works. They definitely seem to not want any third party clients accessing gmail though. Otherwise they would have published a protocol specification a long time ago.