100% grade is 45 degrees. Up one foot and over one foot, up one foot and over one foot, etc. (points to make a 45* line)
A 50% grade would consist of points on a line that ran like this: up one foot, over two feet, up one foot, over two feet. The slope is therefore more gradual at 22.5 degrees. Many of these tests are conducted on "proving grounds" with concrete or asphalt driving surfaces. Still impressive for such a large truck but not real world examples for military, agricultural or utility trucks IMO.
100 percent is nowhere near vertical, in fact its only half way there
Don't confuse percentage of grade rise with degrees, where 90 degrees is vertical, etc.