Prosecutors said Gallo's blood-alcohol level was almost three times the legal limit at the time of the crash. Gallo is charged with three counts of second-degree murder.
Adenhart, a 22-year-old right-hander with a promising career ahead of him, had pitched six scoreless innings during a home game against the Oakland Athletics the night of the accident.
He and three friends were reportedly on their way to a dance club when their car was hit. Adenhart's friends Henry Pearson and Courtney Stewart were also killed in the crash. A third friend was seriously injured.
If convicted of all charges, Gallo could be sentenced to more than 50 years in prison.
The next court hearing is July 26.
CNS and AP contributed to this report.