After the slate was approved Wednesday night at the county convention at Wisdom Lane Middle School in Levittown, Gross beamed alongside town board members De Giorgio and fellow Republican Angelo Ferrara.
Both women held bouquets of flowers and the trio wore "Elect Dina De Giorgio" buttons.
"I was welcomed when I walked in there," said Gross. "Like an old family member."
Ferraro, who is running for town council, said, "In my mind, this is not a surprise."
The switch, Gross said, was set in motion amid uncertainty about her position on the slate. In April, a spokesman for the North Hempstead Democratic Committee declined to comment about whom it would run for the clerk position. That spokesman identified other Democratic candidates on the party's slate.
"That was a defining moment, my first indication of what was going on," Gross said. "I didn't want to wait around to find what that meant."
Gerard Terry, chair of the committee, declined to comment Wednesday about Gross' switch or who -- and whether -- the party would oppose her.
North Hempstead -- and the town board -- is largely Democratic. De Giorgio herself was a registered Democrat before switching parties in 2011 in her successful challenge of Fred Pollack.
"We're forming a coalition to show the government that bipartisanship can work," said Gross, who was elected in 2007 and 2011, and has a campaign chest worth $18,576.68, according to January filings.
Gross was appointed town clerk in 2007, succeeding Michelle Schimel when she was elected to the State Assembly. Previously, Gross was director of business and tourism.
De Giorgio said she asked Gross to join her ticket shortly before deciding on her own race.
Both recalled bonding at events over the last few months. "I've felt a real kinship with Leslie," De Giorgio said. "She had to think about it, but I'm happy to say it is happening, and she said yes."
Gross, referencing Ferrara, the only other Republican on the town board, said: "After two elections, I kind of weighed all of my options. This one was with two people that I do like and respect."
Frank Moroney, chair of the North Hempstead Republican Committee, said "there is some historic value to this ticket, the way it's configured," adding, "It really is time for a change."
North Hempstead has not had a Republican supervisor since 1989.
Moroney credited De Giorgio and Ferrara with enlisting Gross. "I give a lot of credit to Dina and Angelo for seeing the talent for what it was, for holding those conversations."
Moroney announced two other candidates for the GOP council slate: Anthony Bulzomi, a construction manager from Westbury to challenge Viviana Russell in the first council district; and Jeff Benjamin, an attorney from Great Neck opposing Lee Seeman in District 5.
Moroney said he didn't consider running a Democrat risky.
Moroney said he and Gross met last Friday. Another candidate, whom Moroney declined to name, was to run for the clerk position but was asked to step aside. Instead, Moroney said, he'll run for a county legislative seat.
De Giorgio acknowledged tensions between Gross and town Democrats, calling the relationship "strained."
Gross said she was not "able to give the input or maybe my opinions have not always been appreciated." The town did not respond to requests for comment.
"I guess I feel right now very appreciated for the work I do," Gross said. "This was not something I ever considered until they showed such excitement for me."