Judge Gillian Abramson sentenced him to life in prison without parole, which is automatic in first-degree murder cases.
She also scolded him, saying "infinity is not enough jail time for you."
WBZ-TV's Jim Armstrong reports from Nashua
Gribble had admitted to killing Kimberly Cates and nearly killing her 11-year-old daughter Jaimie. The jury was deciding whether Gribble was insane at the time he committed the murder.
WBZ NewsRadio 1030's Mark Katic reports
He was also convicted of attempted murder, conspiracy to commit murder, conspiracy to commit burglary and tampering with witnesses.
Gribble had no reaction as the verdict was read.
After a 15-minute break, he returned to the courtroom in an orange prison jumpsuit for sentencing.
Cates' husband David then read an emotional victim impact statement before the judge handed down the sentence.
"This verdict is not justice," David Cates said. "But I can only hope that justice will find you very soon."
Cates then called Gribble a "worthless coward" and blasted him for all the pain he has caused his daughter Jaimie, who Cates said will "ache" for the rest of her life because of what happened to her and her mother.
After the sentencing, the judge told Jaimie Cates, who appeared in the courtroom for the first time Friday, "this man and the other men involved in this terrible crime can never hurt you again."
Outside of court, Mont Vernon's police chief spoke about the verdict. He said he believes it was the appropriate outcome, but acknowledged things in his town will never be the same. "The sense of safety and security the community had… has been irreparably damaged," Chief Kyle Aspinwall told WBZ NewsRadio 1030.
WBZ NewsRadio 1030's Bernice Corpuz reports
The panel got the case Thursday afternoon and deliberated for about two hours.
They returned to the courtroom Friday with a verdict just after 9 a.m. All of them left without talking to reporters.
In closing arguments, Assistant Attorney General Peter Hinckley emphasized that Gribble was sane because he plotted, chose the weapons and knew to hide evidence and to initially lie about his involvement.
Defense attorney Donna Brown reminded the jury that even depression is grounds to find Gribble legally insane at the time of the crime.
She portrayed Gribble as an abused child who suppressed urges to kill until he fell in with his accomplice, Steven Spader.
for more features.