gnus-default-charset

gnus-default-charset is a variable defined in `gnus.el'.
Its value is

undecided

Documentation:
Default charset assumed to be used when viewing non-ASCII characters.
This variable is overridden on a group-to-group basis by the
`gnus-group-charset-alist' variable and is only used on groups not
covered by that variable.

You can customize this variable.