Half solved.(self reply)
Latest official nvidia-settings package of gentoo is valid for old drivers.
I tried to rollback to 7676, SDI signal was activated.
And then, I got the newer nvidia-settings tarball from nvidia ftp site.
It could be built and could control settings on 8756 driver,
EXCEPT SDI out....
Is the latest nvidia-settings suppoted 8*** driver ?
( Because I want to try RGBA output of X desktop. )