The dross does not adhere as easily on the galvanized zinc coating. However, by dialing in your speed and torch to work distance you can achieve the same cut on mild steel. 99% of the time people tend to cut gauge steel too slow.....and when it produces dross or warps they tend to go even slower! The dross free zone (dfz, a speed range that produces little or no dross, usually near the max penetration speed for a given thickness and process power level). Often...speeds to achieve dfz are above the speed capability of the cnc machine. For cutting thin gauge with plasma you should have a machine with acceleration in the 60 miligee range or higher, and the ability to cut at speeds in the 150 to 350 ipm range while maintaining torch to work distance. Plasma can cut very nicely on these materials....but needs excellent motion control to do so! Hypertherm introduced FineCut consumables a few years ago for some of its air plasma systems...these consumables are designed with higher energy density at lower power levels...allowing extremely good cut quality on materials thinner than 3/16"....at reasonable speeds.
Be careful with the fumes when cutting galvanized!