The new arrival, who was born on Tuesday evening in Monaco's Princess Grace Hospital, has been named Raphaël. His birth was announced by the president of the National Council, Laurent Nouvion, at a late meeting of the High Assembly. "The royal family has a new member," he said.
So far no further details have been revealed.
The little boy is the second Monegasque royal to be born this year: Charlotte's brother Andrea and his wife Tatiana Santo Domingo welcomed their son Sacha in March.
In the run up to the birth, Gad, who has a 12-year-old son named Noé from his relationship with actress Anne Brochet, told People magazine that his 27-year-old girlfriend would be "the best mother in the world," because she is "generous and loving".
Eyebrows were raised when the couple got together in early 2012 because the Frenchman is 15 years Charlotte's senior.
There is, no doubt, however that this is a genuine love match. Just recently, charismatic Gad spoke about what drew them together and described his partner as "a smart and very intelligent girl who has a lot of humour".
"You can talk about philosophy and make stupid jokes with her," said the screen talent. "We go to see art or she will go crazy over a TV show and laugh out loud with me about a stupid gag."
He added: "We have real conversations and share things. I'm the happiest man – really, I am. I am lucky to be with her."
Her family have also accepted the relationship, with the actor adding that Princess Caroline laughs at his jokes.
Whether the new parents will marry remains to be seen. He recently scotched rumours of an engagement but said "maybe one day".