Mild steel is the universal base material for steel gates, fences, and balustrades. Low carbon content (under 0.25%) makes it tough, weldable with any common process (MIG, TIG, stick), and forgiving on cuts and bends.
For gate work, the structural grade is AS/NZS 1163 C350 (350 MPa minimum yield). Plate steel is AS/NZS 3678 grade 250 or 300. Both are stocked galvanised by major Australian merchants for gate-trade volumes.
The weakness is corrosion. Bare mild steel rusts the moment it sees rain. Galvanising, powder coating, or both are mandatory for any outdoor mild-steel work intended to last more than a few years.