The following command can delete a folder and/or all its contents in Gnus.
To just delete a folder:
G RET DEL
The "G" letter means capital G. the RET means Enter and the DEL means Del or Backspace in most of the computers this era.
This combination will only delete a folder, but it will not delete the contents inside. The contents will be, however, moved to another folder such as INBOX or Archive, or something like that.
To delete a folder and all its contents:
C-u G RET DEL
In this combination of command. The first "C" is not the capital letter "C". It means Ctrl in most of the computers nowadays. So C-u means that you press Ctrl + u.
Have fun. Be free.