"We are going to be the last mass shooting ... We are going to change the law," she vowed — slamming the fact 19-year-old gunman Nikolas Cruz was able to legally buy a semiautomatic firearm despite a history of troubling and violent behaviour.
"The question on whether or not people should be allowed to own an automatic weapon is not a political one. It is question of life or death and it needs to stop being a question of politics," Gonzalez told AFP following her speech.
In Washington, the political response has made clear that the powerful NRA pro-gun lobby remains formidable, while Trump himself suggested the root cause of mass shootings was a crisis of mental health — making no mention of gun control.
"If the president wants to come up to me and tell me to my face that it was a terrible tragedy and ... how nothing is going to be done about it, I'm going to happily ask him how much money he received from the National Rifle Association," Gonzalez said in her impassioned address.
"It doesn't matter because I already know. Thirty million," she said, citing the sum spent by the NRA to support Trump's election bid and defeat Hillary Clinton.
She then ran through a list of the pro-gun lobby's talking points — for example, that "a good guy with a gun stops a bad guy with a gun," that no law could ever stop a madman intent on killing — answering each argument with "We call BS."
The young woman's powerful address immediately went viral, with her name a top trending topic on Twitter.
Trump tweeted a day after the massacre that neighbours and fellow students had failed to flag Cruz to the authorities.
"We did," Gonzalez fired back, her voice shaking with emotion as she insisted the community had done its best to raise the alarm.
"Time and time again. Since he was in middle school. It was no surprise to anyone who knew him to hear that he was the shooter."
US authorities have come under mounting scrutiny for failing to act on a series of warning signs.
The FBI admitted Friday it received a chilling warning in January from a tipster who said Cruz could be planning a mass shooting, but that agents failed to follow up.
Cruz was also known to local police after his mother repeatedly reported him for violent outbursts, while records obtained by the South Florida Sun Sentinel show authorities investigated Cruz in 2016 after he cut his arms on messaging app Snapchat and threatened to buy a gun.
The newspaper, citing Department of Children and Family Services documents, said the investigation came four days after Cruz turned 18 — legally an adult, and thus able to buy a firearm.
Investigators said there were "some implications" for the teen's safety, but concluded that his "final level of risk is low as (he) resides with his mother, attends school and receives counselling" as an outpatient at a mental health centre, the Sun Sentinel said.
Cruz later passed a background check, allowing him in February 2017 to buy the AR-15 rifle used in the massacre.
Trump spoke by phone Saturday with the Parkland mayor, the county commissioner and the principal of Marjory Stoneman Douglas High School to express his condolences and offer his support.
He then turned political in a tweet Saturday night, asking "why didn't the Democrats pass gun control legislation when they had both the House & Senate during the Obama Administration. Because they didn't want to, and now they just talk!"
Trump is staunchly opposed to additional restrictions on guns or gun ownership, but Vice President Mike Pence said at an event in Dallas the president would make school safety "a top priority" when he meets with governors of US states in the coming days.
"Let's pray for wisdom. For all in positions of authority that we might find a way to come together as a nation to confront and end this evil in our time once and for all," Pence said.
