First of all, MAZAL TOV!
Secondly, I highly recommend getting a hold of the “Chosson Kallah Guide” available in various locations. It’s very useful for preparing for the Chasunah, has articles, addresses, coupons, and tons more. The Guide should tell you where you can go for gowns and for everything.
Third, as far as free gowns, I don’t know Brooklyn, but Kaila Stern’s Gemach in Lakewood has gorgeous gowns for no charge (there is a suggested donation, though, which is not high at all, and no one feels pressured to give it.)