I don’t think that there is any such plea.
An acceptance of insanity by the court precludes guilt.
I don't think there's a plea of “guilty by reason of insanity,” but I did read the other day that in Arizona, pleas of not guilty by reason of insanity are not permitted.
There is a verdict of “guilty, but insane.” The upshot is the person convicted in this manner is committed to a mental hospital until such time, if it ever comes about, that he recovers from his insanity. At that time, he is sent to prison.