What's steel Gauge meaning ?

The term “gauge” refers to the thickness of the steel, and it plays a significant role in determining the strength, durability, and performance of a structure.

Where did this unit of measurement come from?
This unit of measurement and its name came from the Industrial Revolution and the British wire industry, which had no common unit to measure thickness. Workers drawing wire would calculate the diameter based on the number of times it was drawn, which became the gauge (so the higher the gauge, the thinner the material). The more times it was drawn, the thinner the wire became.

Why are they used instead of inches or millimeters?
As mentioned above, the gauge was created to act as a universal or standard unit of thickness. Since then, this system of classification has stuck (and even expanded to products like needles) as an easy way to identify standard sizes of products such as wiring diameter, sheet thickness, and tube wall thickness. So, instead of saying that you need steel sheets that are 0.0478 inches thick, you can simply request 18GA sheets.

Picking the right gauge
The ideal gauge for your steel greatly depends on the application, so there are some key factors to keep in mind. Thicker steel will of course provide more strength, but also has decreased flexibility and a wider bend radius. For example, a fabricator or supplier might recommend switching from 14GA to 16GA sheet — to tighten a bend radius or save weight. Rigid, edged objects can use thicker (or lower gauge) steel, while more flexible or curved objects will likely need thinner (or higher gauge) steel to accommodate this.