Prosecutors argued Gaston, an alleged gang member, fired at two people on a South Side sidewalk two years ago, killing Valadez.
"My nephew, who was born three months after my brother was murdered, will never know what his daddy's hugs and kisses feel like," said Brenda Valadez, sister, after the verdict was read.
Closing arguments got under way Wednesday afternoon before a packed courtroom and a verdict was reached just after 9 p.m. Sitting in the front row was Chicago Police Superintendent Garry McCarthy and his first deputy.
Valadez was known as "Alex" to his friends and family.
Valadez's parents, three siblings, fiancee and the 27-year-old's family of police officers didn's miss a day of the eight-day trial. As hard as it was, they listened to the details of how Valadez was shot to death while investigating a call of shots fired in the Englewood neighborhood in the early morning hours of June 1, 2009.
Prosecutors said Gaston and his two friends were out for revenge against rival gang members when they opened fire while driving down the 6000-block of South Hermitage.
In his closing argument, prosecutor Frank Merrick said Valadez "never had a chance. He never saw it coming. And that is first degree murder."
While three guns, including the murder weapon were found in Gaston's car, the defense claimed those guns were planted by Chicago police officers.
The lead prosecutor in the case was Cook County State's Attorney Anita Alvarez. This is the first case Alvarez has tried since becoming state's attorney. The last time she was a prosecutor before a jury trial was in 2005.
"I took this case because it was so horrific that he was gunned down in such a senseless way, I got on the case back then in 2009, and I thought it would be important for me as the state's attorney to handle this case," said Alvarez.
Gaston faces up to 101 years in prison.
Two other men charged are awaiting trial in Valadez's killing.