Garlic salt and garlic powder have very different consistencies, since garlic powder has the texture of flour and garlic salt is much more coarse. Garlic powder is best used as a rub for meats because it blends in easily and has a more intense garlic flavor. Garlic powder is actually dehydrated, ground up garlic, making it very potent in smaller quantities, compared to garlic salt.
Garlic salt can be used as a substitute for garlic powder, but must be used in a larger quantity and requires the cook to reduce the amount of salt in the recipe. Garlic salt is a blend of garlic, salt and garlic oil.