Gnus, by default, handles all its groups using the same approach. In particular, you do not do anything explicitly to delete messages. But, no, it means that old messages are expired according to some scheme or other. Expiring Mail . 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.