The 22-year-old Gallo is charged with three counts of murder and other felonies in the April 9 crash.
Also killed in the April 9 crash were 20-year-old Courtney Stewart, a student and former cheerleader at Cal State Fullerton, and Henry Pearson, 25, a law student working toward becoming a sports agent. Jon Wilhite, 24, of Manhattan Beach, Calif. was critically injured in the crash.
Gallo watched from inside a security enclosure as Longwith entered the pleas. About 15 members of Gallo's family were in court.
While waiting for Gallo to appear, Wilhite, the lone member of Adenhart's group to survive, began to sob. Wilhite, who had critical injuries in the wreck, was embraced by friends and family.
Longwith said later in an interview that he will seek a change of venue because he believes Gallo cannot get a fair trial in Orange County, home of the Angels and the California State University, Fullerton, baseball team for which Wilhite had played.
The attorney noted that in another courthouse where Gallo made a previous appearance, an Angels poster was in the hallway leading to the courtroom. Longwith also said there have been death threats against Gallo and himself via the Internet, phone and in a letter.
"I think people are venting and it's just evidence of the passion people have in this case. I don't think it's a true threat," he said, noting he had not brought it to the district attorney's attention.
Deputy District Attorney Susan Price disputed the need for moving the case.
"We don't believe that the defendant will be any more prejudiced in this county than in any other county," she said.
She said Gallo faces a mandatory minimum sentence of 45 years to life in prison if convicted of the three second-degree murder charges. If convicted of all charges, the minimum would be 54 years and eight months to life.
Police have said Gallo had nearly triple the legal blood-alcohol level when his minivan ran a red light and crashed into the car -- he was reportedly going about 65 mph in a 35 mph zone.
Adenhart, who was originally from Williamsport Maryland, died only hours after he had pitched six scoreless innings for the Angels in his major league regular season debut.