From a real reliable source, Zegna does make the suits for Armani, Gucci, and Tom Ford. As to what lines, I didn't ask. For Armani, it could go all the way up to the Georgio's. Lower lines probably aren't (like Collezione).
My wife speaks fluent Italian and she says the g is silent (I always tease her by pronounciating the "g" in all sorts of Italian word like Pulgiese. Her dialect is more Napolitane, but she says it's pretty consistent with the silent g in all of Italy and on Ischia, where she lived.
Funny one is how people also butcher Isaia. They keep adding an extra "i" in the pronunciation.