About Gradians
A gradian (also called a gon or grad) is 1/400 of a circle, so a right angle is exactly 100 gradians. It was introduced in France alongside the metric system as a decimal alternative to degrees.
Gradians are mostly found on surveying instruments in continental Europe, which are often marked in gons. One gradian is 0.9 degrees, or π/200 radians.
On a gradian scale a quarter turn is 100 and a half turn is 200, so right angles come out as round numbers. The catch is that 30° and 60° become 33.33 and 66.67 gradians.
About UK Milliradians
The UK milliradian is one thousandth of a radian, or 0.0573°, so 6,283 of them fit in a circle. At 1 meter it spans 1mm.
An object that fills 1 milliradian of your view is 1 meter wide at 1km, or 1mm wide at 1 meter. Divide the width by the angle to get the range – a 1.8m person covering 3 milliradians is 600m away.
Degrees don't give you that shortcut: at 1km one degree is about 17.45m wide, which is nobody's idea of a round number.