Every scientific calculator I've come across has a DRG button that controls whether trigonometric functions use units of degrees, radians or grads yet I've never seen or heard of any system that actually uses grads. When is the grads setting useful?
Grads were developed by the French as part of the metric system and have been referred to as the metric degree.
In some countries in Europe surveyors use grads instead of degrees. The other use is by French artillery units who have used grads for decades.
Gradians are commonly used in land surveying. I only know this because I made the mistake of asking the same question of a Civil Engineer.
Grade is often used for inclination, such as looking at how steep a road is. The units are not as "crazy" as they seem, when you think of grade as a percentage. In other words, a hill can go down at anything from 0% (flat) to 100% (45 deg angle). (rise/run) Based on some comments added below, it is not clear if the G on this calculator is actually grade or not.