Michio Hirano, a professor of neurology at the Columbia University Medical Centre in New York, is in the UK to examine 11-month-old Charlie.
Great Ormond Street Hospital has said clinical staff are “on hand to facilitate the visit” and Dr Hirano has been given an honorary contract, giving him full access to the baby’s medical records.
His meeting with specialists is the latest stage of a court battle over the best course of action for Charlie, who suffers from a rare genetic condition and has brain damage.
A judge ruled that Charlie’s mother Connie Yates could be present for the gathering.
Ms Yates and Charlie’s father, Chris Gard, want to be allowed to take their son to New York to undergo a trial therapy overseen by Dr Hirano.
But specialists at Great Ormond Street say the therapy is experimental and will not help Charlie.
They do not believe that Charlie has any chance of surviving his critical illness and that life-support treatment for the child should stop.
Dr Hirano has said he believes there is now a better chance the treatment would produce a meaningful improvement than there was when he gave evidence three months earlier.
The couple, who are in their 30s and come from Bedfont, west London, want Mr Justice Francis to re-open the case in light of the new evidence.
Preliminary hearings were held at the Family Division of the High Court in London over the last week.
More are scheduled for later this month following this week’s gathering of medical experts.
In April a judge ruled in favour of Great Ormond Street, saying Charlie should be allowed to die with dignity.
The parents have previously lost battles against the hospital in the High Court, Court of Appeal and Supreme Court in London over Charlie’s treatment.
The European Court of Human Rights also decided not to intervene in the case.
Alasdair Seton-Marsden, who represents Charlie Gard’s family to the press, has said baby Charlie has become “a prisoner of the state”.
He argued that if the couple were rich instead of “ordinary people” their child would be free to receive treatment in the US.
Litigation is due to resume after Dr Hirano’s meetings – this is likely to be Tuesday or Wednesday.