Gardner lived in Frederick, MD and Giordano is the only suspect in her disappearance.
After a hearing on Monday, a judge decided the prosecution didn't have enough evidence to keep Giordano in jail.
Investigators consider him a suspect, because they say he tried to cash in on the travel insurance policy taken out on Gardner. The policy was worth $1.5 million.
After four months of searching for Gardner, officials in Aruba believe she is dead.
Prosecutors were trying to convince the judge to hold Giordano for another 30 days. If that extension was granted, it would have been the final extension to keep him in Aruba.