Rep. Gabrielle Giffords -- Arizona Shooter Jared Loughner PLEADS GUILTY

Jared Loughner -- the guy who shot Rep. Gabrielle Giffords -- has changed his plea to "guilty" after last year's Tucson, AZ shooting, in which he killed 6 people and injured 13 more ... and has agreed to spend SEVEN life sentences in prison.

A plea of "not guilty" had previously been entered on Loughner's behalf -- but today, the judge ruled Loughner was competent to understand the gravity of the charges against him ... thanks to forcible treatment of his schizophrenia.

The shooting took place back on January 8, 2011. Loughner opened fire in a Safeway parking lot, where Rep. Giffords was holding a constituents meeting.

Loughner was originally charged with 49 criminal counts including first-degree murder. Today he pled guilty to 19 counts, including attempted assassination, 1st degree murder of 2 federal employees, and attempted murder of 2 federal employees. The remaining counts were dismissed.

The federal employees Loughner gunned down and killed were U.S. District Court judge John Roll and one of Giffords' staffers, Gabe Zimmerman.

Loughner's plea deal will allow him to avoid the death penalty, but he's agreed to serve seven life sentences in prison ... plus an additional 140 years.

Giffords, who was shot in the head, is slowly recovering. Shortly before Loughner entered his new plea today, Giffords released a statement, saying, "The pain and loss caused by the events of Jan. 8, 2011, are incalculable."