Maybe I'm wrong. For that you have to ask the Italians.
If Gian was a German word (which it ain't) it would be 2 syllables.
When I said that in the trailer it sounds Ok, I meant of course taking into consideration that it was pronounced by an english speaking person.
It is in fact a one syllable word which is short for Gianni which in turn is short for Giovanni (=John). Gian is used only if there are two first names like in this case Gian Maria.
To come back to the pronounciation, one syllable, keep it short and put the accent on the "a" as in "after".