Hi, and welcome. The grade is expressed as a percentage of units of linear movement in elevation divided by the linear movement along a horizonal axis. For example, if you move 8 feet toward a direction, but also rise 3 inches, you get 8' X12" (to get common units between the rise in elevation and the "run" horizontally) equaling 96" and divide your rise of 3" by the figure of 96. That comes to 0.03125, or moving the decimal two places to get hundredths, 3.1%. For the distance travelled horizontally, in other words, the elevation change is positive 3.1%. That's a substantial grade, even for model railroads.
Degrees are an expression of angle. We use them sometimes when we talk about a curve, but not in the modeling world. The big railroads use degrees when plotting out curves as they survey.