Pre-galvanised steel ('pre-galv') is steel sheet or coil that goes through a continuous galvanising line (CGL) before being formed. The zinc coating is around 20 microns, much thinner than hot-dip galvanise after fabrication (60 to 100 microns).
The big advantage is that fabricated pre-galv parts are corrosion-resistant straight off the saw. No second trip to a galvanising bath. Cut edges and weld zones lose their zinc protection, but a touch-up with cold-galv spray or zinc-rich primer covers them adequately for less aggressive environments.
Pre-galv RHS and SHS are common for low-cost gate frames in inland Australia. For coastal C5 environments, hot-dip galvanise after fabrication is the safer call.