It’s the same as wire gauge, but the scale is different per material!
For steel
16g = 1.5mm
22g = .75mm
And so on
Pretty much anything larger than 1/8th of an inch (3.2mm), we just use the measurement (in inches).
Why? Because we don’t have a common measurement unit smaller than an inch. There probably is one, but no one commonly uses it. Coming up with a gauge scale is actually easier than say .0123 inch thick. Say “12 gauge wire” and everyone (who knows about it), knows exactly what you’re talking about.
Pretty much anything below 1/32 (0.8mm) of an inch, we’ll switch to decimals. 0.0001 inch is valid with no common way to make that neater. No such thing as 1 mili-inch.