Gabriella has been living with her grandparents in Tehran and visiting her mother in the notorious Evin Prison since Mrs Zaghari-Ratcliffe was arrested on spying charges more than three years ago.
Mr Ratcliffe held an emotional press conference on Friday with his MP Tulip Siddiq.
Gabriella and those in attendance were treated to sponge cake, which the five-year-old visibly enjoyed, while her father updated journalists on the plight of his wife and how she will cope without her daughter's prison visits.
"I spoke to Nazanin yesterday and she was reasonably distraught," Mr Ratcliffe told reporters at Westminster on Friday.
"One of the things she really didn't want to happen was her daughter to leave while she was still in prison."
Mr Ratcliffe said there is a "real risk" his wife's mental and physical condition will deteriorate now that her support - Gabriella - has left.
"If I'm honest though I think it will deteriorate anyway," he said.
Mr Ratcliffe said upon his daughter's return: "Gabriella came back to us late at night, a bit uncertain seeing those she only remembered from the phone.
"Now she is peacefully sleeping next to me. And I am just watching.
"It has been a long journey to have her home, with bumps right until the end - and I am grateful to all those at the British Embassy and Iranian MFA who helped smooth all those last blockages.
"Of course, the job is not yet done until Nazanin is home," he added.
"It was a hard goodbye for Nazanin and all her family. But let us hope this homecoming unlocks another."
Gabriella spent three and a half years living in Iran, visiting her mother in the notorious Evin Prison when the authorities would allow her to do so.
Her ability to speak English is severely diminished, and she will have to relearn how to communicate with her father, said a family statement.
Jolie King, a dual British-Australian citizen, and her Australian boyfriend Mark Firkin were returned to Australia last week, some three months after being arrested.
Mrs Zaghari-Ratcliffe was arrested in April 2016 and sentenced to five years in jail after being accused of spying, which she vehemently denies.
Labour's Tulip Siddiq said Gabriella's parents have made a decision that "no family" should have to make.
"It is heart-warming to see Gabriella reunited with her father after 1,300 days in Iran, but heart-breaking that she is separated from her mother Nazanin," Ms Siddiq said.