Harris is the wrong candidate to be heralding a record of criminal justice reform because of her record as a former California state attorney general, Gabbard claimed Thursday on "Tucker Carlson Tonight."
"Voters deserve to know the truth," she said.
"The decision that the American people have before them is a critical one. The decision that Democrats have before them about who their nominee should be is a very important one.
"They deserve to know the truth about the candidates that are before them, and especially on the issue of criminal justice reform that’s impacting so many people across this country because it is so broken and it is an unjust criminal justice system."
During Wednesday's debate, the Gabbard challenged Harris' record, claiming she imprisoned hundreds of people for marijuana violations then made light of her past reported use of cannabis during a more recent interview.
"Senator Harris says she's proud of her record as a prosecutor and that she'll be a prosecutor president, but I'm deeply concerned about this record," Gabbard said.
"There are too many examples to cite but she put over 1,500 people in jail for marijuana violations and then laughed about it when she was asked if she ever smoked marijuana."
She also blasted Harris for maintaining the cash bail system which, she argued, disproportionately hurt poor people. Gabbard accused Harris of keeping prisoners beyond their sentence in order to use them as "cheap labor" as well as blocking evidence that would have "freed an innocent man from death row."
In a February appearance on New York's "The Breakfast Club" radio show, Harris spoke about her opinion of marijuana.
“I think it gives a lot of people joy. And we need more joy in the world," Harris said at the time.
In her interview with Carlson, Gabbard said the California Democrat owed an apology to some of those she helped put behind bars.
She added the state AG role offered an opportunity for Harris to effect change in criminal justice reform.
"Senator Kamala Harris was in a position to be able to make a difference on that when she was serving as Attorney General," she said.
"As I pointed out last night, she did not fulfill that promise that she claims to have, which is to be a champion for the people, a champion for the oppressed. In fact, she did the exact opposite.
"She made decisions that ended up hurting people."
In addition, Gabbard said Harris' campaign "smeared" her following the debate.
"The responses from her and her campaign... was basically to lob cheap smears and basically say that I'm a traitor or a foreign agent or a Trojan horse or whatever."
"This is concerning on many levels about how a Senator Harris would govern as president, that if there are those who are critics of her or her record -- you know for me I’m a two-time veteran of Middle East deployments, I’m a soldier currently serving in the Army, I’m a sitting member of Congress -- and she’s smearing me in this way. Imagine what she would do to anyone else."
Fox News' Sam Dorman and Jennifer Earl contributed to this report.