Charlton was one of the United players Gregg pulled from the wreckage of the burning plane that crashed at Munich airport in 1958.
Former United manager Ferguson and former striker Denis Law also attended the service in Coleraine, in Northern Ireland.
Former Northern Ireland international Gregg, who was named the best goalkeeper of the 1958 World Cup, died earlier this week aged 87.
He will be forever associated with the tragedy in Munich when a plane carrying Matt Busby's young United side back from a European game against Red Star Belgrade crashed in the snow after a refuelling stop.