The actor was driving an SUV in Beverly Hills last December when it crashed into a tree, killing Vahagn Setian and injuring two 15-year-old girls. Tests showed he had a blood-alcohol level more than twice the legal limit and was under the influence of cocaine.
Garrison pleaded guilty in May to a felony charge of vehicular manslaughter without gross negligence, as well as driving under the influence and providing alcohol to a minor.
Prosecutors claimed he met the teenagers in a grocery store and accompanied them to a party, where he drank vodka before driving them back to the shop for more alcohol.
Vahagn Setian's friends and family presented the judge with a 3,600-signature petition on Tuesday, demanding that the actor receive the maximum sentence of six years and eight months.
Garrison's attorney Harland Braun described him as "philosophical", saying: "He's going to survive. He'll be out in about 18 months.... He said to me: 'Look, there was an accident. Someone's dead. I'm alive. I'm the lucky one'. "
The 27-year-old played David "Tweener" Apolskis in the hit Fox drama.