These are both terms used to measurements for a liner's thickness! Mils are regulated while a gauge is more of an approximation.
GA Mil is an actual unit of measurement and gauge is more of an estimate based on findings. Mil represents Gauge one thousandth of an inch.
- Mil is a true regulated form of measuring the thickness of a liner.
- Gauge, in vinyl liners, is not regulated and is more of an estimated thickness.