Remove the bag of chicken from the fridge about 30-40 minutes before you are ready to grill. Heat your outdoor grill to 425-450°.
While your grill is heating-up, pull the reserved marinade from the fridge (the portion that you kept separate from the chicken) and transfer it to a small saucepan over medium heat. Bring it to a vigorous bubble, then reduce heat to medium-low and let it simmer (stirring frequently) for about 20 minutes until it thickens slightly and becomes glaze-like. Once done, turn off the heat and leave the glaze to stay warm in the pan until you are ready to serve. (See TIP B for what to do if it becomes too thick.)
TIP A: If you follow my recipes, you've probably noticed that I'm a bit sporadic in when I recommend the use of granulated garlic (a.k.a. coarse garlic powder) vs. whole garlic grated or minced. They can be interchangeable—however not in the same quantity. In most sauces and marinades, using a high quality granulated garlic in the place of minced or grated fresh garlic is perfectly fine—in fact, sometimes even preferred. Granulated garlic is often more intense in flavor, but because it is ground and dry, it easily blends into liquid mixtures. Fresh garlic has a more complex and savory flavor that typically mellows when you cook it. I'm a fan of both. The key things to remember are: 1) if you substitute granulated garlic for minced or grated fresh garlic, use less (I recommend 1/2 teaspoon for every 1-2 minced or grated cloves). And 2) if you're worried about "over-doing it" with garlic flavor, then use minced or grated fresh garlic vs. granulated garlic. This might seem a bit counter-intuitive but remember that fresh garlic mellows in flavor when it cooks—granulated garlic typically does not. Obviously this rule holds only for preparations that involve heating or cooking. You'll want to be much more precise with your quantities when using fresh garlic or granulated garlic in raw instances...but don't be shy!
TIP B: The only tricky thing with preparing a glaze is that once it starts to thicken, it just keeps thickening. Ideally, your timing would be such that once the chicken comes off the grill, your glaze is at the perfect consistency. This never happens. So, go ahead and start make the glaze after preheating the grill and then if you need to loosen it up a bit, just add a couple of teaspoons of hot water and stir vigorously. It will be perfect!