Ghoulam believes PSG will struggle to defend their Champions League crown without the Italian.
He told Il Corriere dello Sport: "Donnarumma? PSG made a huge mistake and showed disrespect to Gigio.
"They wanted to please Luis Enrique because he won, but you can't do this to a totemic player who was crucial to the victories themselves.
"Gigio is one of the two or three best goalkeepers in the world and will be among the top ten for the Ballon d'Or, but what would have happened if (Pep) Guardiola hadn't arrived?
"I can't explain it because it's an inexplicable choice. It's stupid!"
On Napoli this season, he said: "I expected a little more from the transfer market. I don't know if they'll be able to manage three competitions: the club has invested heavily, but the transfer market is full of opportunities that don't always present themselves.
"They certainly have a lot of champions, and Conte is incredible: he performed a miracle by winning the Scudetto with a very short squad and many injuries, and he continues to adapt his ideas to the players. Now he's combining McTominay and De Bruyne, maintaining a balance. He's superb."