If Gillum wins, he will be the first Democratic governor elected by Floridians in 20 years.
The outgoing governor, Rick Scott, was term limited.
This year's totals far exceed those of 2014 midterms, but are still short of the 6.6 million who voted ahead of Election Day in 2016. He conceded to DeSantis, who has been an outspoken supporter of President Trump, saying he still "believes in the long run, good always wins over evil".
A recount is already planned for the Florida Senate race, where Republican Gov. Rick Scott now leads Democratic Sen. And while Republicans swept Florida's statewide races, Democrats again crept closer to winning, and may get a boost going forward from a new constitutional amendment that will restore felons' voting rights.
All told, Democrats had flipped at least seven Republican-held governorships without suffering any losses as of early Wednesday morning.
"We can not win the state of Florida if we continue hemorrhaging those kinds of margins in red counties", Schale said.
In Georgia, Democrat Stacey Abrams, 44, was vying to become the nation's first black female governor against Republican Brian Kemp, the state's secretary of state. With all votes counted, Mr Scott had 50.2 per cent of the vote, a lead of 0.4 per cent.More news: U.S. Iran Sanctions Take Effect, No Word Yet On Waiver Countries
"I want you to know that in spite of our congratulating him on his victory this evening, nothing that we believe in is compromised", Gillum told his supporters Tuesday evening. "We are still on the verge of history, and the best is yet to come".
The day before the election, when Democrats usually make a last pitch in South Florida cities like Miami, Fort Lauderdale and West Palm Beach, Gillum spent his time in small north Florida enclaves. Michigan, Wisconsin and Pennsylvania - where Democratic incumbent Tom Wolf easily won re-election on Tuesday - are typically Democratic states that swung to Trump in 2016, handing him the presidency.
At the polls Tuesday, many voters said Trump's upset of Hillary Clinton and his support for DeSantis motivated them to cast ballots.
The gubernatorial race was a study in contrasts: a straight-talking, unapologetically progressive, black nominee running against a Tea Party figure who released a campaign ad in which his young children built Trump's fabled border wall out of toy bricks.
Governors and hundreds of legislators elected this year will be in office when congressional districts are redrawn after the 2020 Census. "Now the thing is, will those people register and will they vote".
But the Democrats were trailing in redder areas of the state. Democrats now have complete control of state government in Colorado, New York, Illinois, Maine and New Mexico.
This has been considered one of the most controversial elections after the passing an "exact match" policy, which requires voter details to match information on the state's database. They haven't controlled the state House or Senate since the mid-1990s.