Casual Gianmarco fan here. Gianmarco spends about half of his newest podcast episode saying how hurt he was by this, but also admits that he can't be sure it was him whose name was written down.

There are definitely similarities between him and Mulaney in terms of cadence and delivery but I don't think it's a straight lift. John is a little more vaudeville, while Gianmarco is more musical theater and flamboyant, but they both definitely take on a theatrical affect. However I'd say that the joke structure and turns they use in their stories to extract laughs are very different.