He met with Charlie’s parents, Connie Yates and Chris Gard, and also examined the 11-month-old who remains on life support in the children’s ward.
The American doctor has waded into the saga as a last-ditch attempt to save the little boy’s life.
So far three UK courts and the European Court of Human Rights (ECHR) have all backed the hospital’s opinion that his life support should be switched off.
Dr Hirano will spend time with the youngster and discuss his condition with doctors treating him and specialists.
He has been given an honorary contract giving him full access to Charlie’s medical records.
Dr Hirano believes the nucleoside therapy could improve Charlie’s muscular strength and that there is a “small but significant” chance it would also help brain functions.
The court battle has raged over Charlie, who was born last August with rare genetic condition mitochondrial depletion syndrome, and is also brain damaged and has muscle weakness.
Doctors at the hospital said in a statement: “His depletive genetic disorder leaves him with no muscle function at all now and deprived of his senses, unable to breath and, so far as can be discerned after many months of encephalopathy, without any awareness.
“At the moment, he is on a low dose of oral morphine.
“It has been and remains the unanimous view of all of those caring for Charlie at Great Ormond Street that withdrawal of ventilation and palliative care are all that the hospital can offer him consistent with his welfare.
“That is because in the view of his treating team and all those from whom GOSH obtained second opinions, he has no quality of life and no real prospect of any quality of life.
The situation surrounding Charlie has grown more complex after the Pope and president Donald Trump also became involved.
Mr Trump tweeted: “If we can help little #CharlieGard, as per our friends in the UK and the Pope, we would be delighted to do so.”
His parents, who have battled the courts every step of the way, previously said: “We are utterly heartbroken, spending our last precious hours with our baby boy.
“We’re not allowed to choose if our son lives and we’re not allowed to choose when or where Charlie dies.
Charlie’s case has been publicised globally, with more than £1.3 million raised by the public to help with his treatment.