gnus-signature-separator

gnus-signature-separator is a variable defined in `gnus-art.el'.
Its value is

("^-- $" "^-- *$")

Documentation:
Regexp matching signature separator.
This can also be a list of regexps. In that case, it will be checked
from head to tail looking for a separator. Searches will be done from
the end of the buffer.

You can customize this variable.