Hot-dip galvanising (HDG) happens after fabrication. The welded gate or panel goes through degrease, acid clean, flux, then dip into a bath of molten zinc at about 450 degrees C. The reaction between zinc and the steel surface produces metallurgically-bonded zinc-iron intermetallic layers topped with pure zinc, far tougher than electroplated zinc.
Coating thickness is governed by AS/NZS 4680. The standard calls for at least 85 microns on steel over 6 mm thick and at least 70 microns on steel 3 to 6 mm. Typical service life in inland Australia is 50 years or more before first maintenance. In coastal C5 zones, plan on around 10 years before duplex re-coating.
A few things to remember when designing for HDG. Sealed cavities need vent holes. Provide drainage so dip and rinse fluids can run out. Detail welds for full penetration so molten zinc wicks into the joint instead of pinholing at it.