It so happens I have a small electronic scale precise to 0.1 grams that I use for measuring out seasonings in sausage making. I’ve done the measurements before for a variety of herbs and spices and I found approximately…
1 teaspoon ground black pepper (the kind that comes finely ground, not coarse ground from your pepper mill) => 2.5 grams, so 10 grams is 4 teaspoons
1 teaspoon mustard powder => 2 grams, so 100 grams is 50 teaspoons (1.04 cups). This quantity doesn’t seem proportionate to the other seasonings.
1 teaspoon regular table salt (not kosher salt) => 6 grams so 20 grams is 3.33 teaspoons.
I assume that by “gr” the recipe mean grams. If it’s an old-timey recipe, I suppose “gr” might refer to grains. There are about 15.4 grains in a gram.