"gently said" is correct and usable in written English.
You can use it to indicate that someone spoke in a gentle, quiet, or soft manner. For example: "I'm sorry," he gently said.
But Professor Gelernter gently said, "Joe, I've been through worse".
He looked at his visitor and gently said: "You should leave now.
We've both casually, gently said we'd love to work together again.
"I said: 'Give me the right' and I shook it – but gently," said Ranieri.
("He was very aggressive, to put it gently," said former Representative Herman Badillo, who worked on that campaign).
"I really thought maybe they [Maryland] didn't want the arrangement, and they wanted to let us down gently," said a person familiar with the talks.
An E.M.T. guy woke the kid up gently, saying, "Terry?
"Begin to breathe now, easily and gently," says Dr Oakley.
An usher appeared and shook him gently, saying at the same time.
Mr. Obama pushed back — gently — saying that "all the parties involved have to take seriously obligations they have previously agreed to".
Hannah turned him down gently, saying that her mum always said to never work with pleasure.
