Let's cut right to the chase: Garlic powder is simply dried and ground garlic cloves. As it's a dried product, the flavor is more concentrated, and only about ¼ teaspoon of the product is needed to get the same flavor result as 1 clove of fresh garlic.
Garlic salt, on the other hand, is salt (usually, flaky kosher or sea salt) mixed with garlic powder at about a 3-to-1 salt to garlic ratio.
How To Use Garlic Powder
Garlic powder is a super easy way to imbue soups, stews, stocks, sauces, and more with the perfect amount of garlic flavor that you desire. As it's not a fresh clove, just a whisper of an ⅛ teaspoon can be enough to get the perfect hint of garlic in a dressing, or an item that isn't getting cooked in which raw garlic would be overpowering. I do also recognize using a fresh clove can make it harder to control exact flavor amounts, especially when you start cutting into the garlic. Opening up the surface area of a garlic clove only opens up (or strengthens) the garlic flavor more, which can be too much for some people, especially raw. For that reason, garlic powder is a really nice substitute for fresh garlic.
How To Use Garlic Salt
If you're using garlic salt, there are two elements to consider: The amount of garlic you're going to add to your food can't be measured exactly (as it's not perfectly distributed throughout the salt mix) and, you're also adding salt, so things are going to get too salty before they get too garlicky. A really good use of garlic salt is in a dish where you're adding a set amount of salt anyway, and want a little extra boost of flavor. Something like fried eggs (dusted on top before finishing), a savory biscuit recipe (garlic and chive biscuit, anyone?), or in a pasta sauce.
Garlic Powder Vs. Garlic Salt
If you're reaching for a garlic product because you want garlic flavor, garlic powder is your best bet. If you're looking for a touch of bonus flavor in your recipes, garlic salt is a great substitute.
But, for this reason alone, I have a bone to pick with garlic salt. Why make a product that inhibits my ability to add pungent, breath-ruining, allium flavor to my heart's content? This isn't like that celery salt that's lurking in the back of your cabinet (how much celery flavor could one want, truly?) or a smoked sea salt that's all about imparting just a touch of what can be a super overpowering flavor note. Garlic salt feels like it's really not about the garlic at all. Chicken With 40 Cloves of Garlic is my North Star when it comes to garlic recipes, so a garlic salt is a bit of a waste in my eyes. Not that I'm making every single meal with garlic (there's only so much mouthwash a girl can have), but if I am pulling out garlic for a recipe, I want to be able to taste it. With garlic salt, I'm sorely missing those allium notes.
I fully admit that garlic isn't everyone's favorite, and a garlic salt can be the perfect solution for someone who fears overpowering a dish when even a hint of garlic feels like 40-cloves-of-garlic-levels of garlic. To each their own, but I will be over here happily dumping as much garlic powder as I can into my food.