Grampshell increases internal spawn roll for all resources by 30%. Since spawns are probabilistic, you will have to compare them by their averages, not single runs.
For example, pick a specific zone and resource type, let's say Soil and Quartz. Do N number of runs with and without Grampshell. Calculate total Quartz for each run and compare the averages (total number of Quartz across all runs/N). The higher the number of runs (N), the more accurate your results will be. The average for Grampshell runs should have a value of 1.3x the average of normal runs.