The grade (measured in percentage) is merely a measure of rise over run. A rise of 4" over 100" horizontal is 4%. 1" over 100" is 1%. This is the same whether G scale, Z scale, or full size. A 45-degree incline would be a 100% grade.
The practical maximum grade for G scale (and the smaller scales) is between 3 and 4%. Much steeper than that, and your train lengths really start to get short, or you need to break out the helpers. Obviously, if you can get away with keeping your track level, do so--especially on sidings where cars would otherwise tend to roll downhill if not coupled to a locomotive.
The simplest way to check the grade of existing track is to use the Craftsman [Sears] digital level. The level has degree and % of grade readouts. It also has a lazer beam.
Here is illustrated how I use it on a boxcar or flatcar for instant grade % readouts [ 1.3% grade shown].
If the track isn't placed yet, then set the level on the ground and adjust it to the grade desired. Then shoot the lazer beam forward, creating an even line of sight for the desired grade. If needed, then the level could be raised, showing how much fill is needed for a given grade.