What many Gnus users find, after using it a while for both news and mail, is that the transport mechanism has very little to do with how they want to treat a message. Many people subscribe to several mailing lists. These are transported via SMTP, and are therefore mail. But we might go for weeks without answering, or even reading these messages very carefully. We may not need to save them because if we should need to read one again, they are archived somewhere else.
Some people have local news groups which have only a handful of readers.
These are transported via NNTP, and are therefore news. But we may need to read and answer a large fraction of the messages very carefully in order to do our work. And there may not be an archive, so we may need to save the interesting messages the same way we would personal mail. The important distinction turns out to be not the transport mechanism, but other factors such as how interested we are in the subject matter, or how easy it is to retrieve the message if we need to read it again. It is possible to whip Gnus into a more mailreaderly being, but, as said before, it's not easy.