In its most basic form it is made by combining 3 parts salt and 1 part dried garlic powder by volume, or 6 parts salt and 1 part garlic powder by weight.
From this we know that nominal 'Garlic Salt' is 1/7 Garlic and 6/7 Salt by weight.
Example for 1.25 grams:
1.25 x 6/7 = 1.071 grams Salt 1.25 x 1/7 = 0.179 grams Garlic Powder