Adem Yilmaz, 31, a Turkish citizen, was given 11 years, while Atilla Selek, a 25-year-old German of Turkish origin, was sentenced to five years.
During the 10-month trial, all four admitted to belonging to a terrorist organisation, plotting murder and preparing explosive devices.
Schneider also admitted to attempted murder for grabbing a handgun from a police officer while attempting to evade capture and firing a shot. No-one was wounded in the incident.
He, Gelowicz and Selek renounced extremism and described their actions as a "mistake".
Vehicles packed with explosives were to be used to bomb Ramstein Air Base
Announcing the verdict, Judge Ottmar Breidling said the men had dreamed of "mounting a second 11 September 2001".
"If the accused had managed to do what they planned, it would have led to a monstrous bloodbath, primarily among US army personnel and also civilians," he added.
The judge added that there were now "many impressionable young men and men who have already been led astray, ready to kill for notions of jihad".
"Violent Islamism has penetrated our society and turned young men against it."
Known as the "Sauerland group", after the area of western Germany where three of them were arrested in 2007, the men had trained at camps in Pakistan and procured some 700kg (1,500lbs) of chemicals to produce 410kg (900lbs) of explosives, prosecutors said.
Such a quantity would have been 100 times the amount used in the 2005 London bombings, which killed more than 50 people.
The four men had planned to produce 410kg (900lbs) of explosives
They had allegedly planned to use vehicles loaded with the explosives to kill or injure large numbers of people at locations visited by Americans, the US military base in Ramstein and Frankfurt airport.
But the security services uncovered the plot in December 2006 and conducted one of the biggest surveillance operations in post-war German history.
The men's movements were monitored around the clock for nine months, until it became clear that they were planning to move their huge stores of hydrogen peroxide and an attack was imminent.
Gelowicz, Schneider and Yilmaz were arrested at a rented cottage in Sauerland on 4 September 2007, while Selek was detained in Turkey in November 2007 and later extradited to Germany.
At the time, they were described as "very dangerous terrorists" with a "profound hatred of US citizens", acting on the orders of an "international network".
The BBC's Steve Rosenberg in Berlin says the participation of Gelowicz and Schneider in the plot has shocked Germans and raised concern that militant groups abroad are actively seeking out and signing up Muslim converts to attack the West.
Germany, which has soldiers in Afghanistan as part of Nato but did not send troops to Iraq, has been largely spared militant attacks.
But nine years ago, it emerged that an al-Qaeda cell had used the city of Hamburg as a base for planning the 11 September attacks.
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