Cut the potatoes in half. If they are larger than 2 inches in diameter, you may need to cut them into quarters. Place potato cut-side down on a surface and pierce through center with wooden skewer. Repeat until all potatoes are skewered.
In a small skillet over medium heat, cook the olive oil, garlic, rosemary, and 1/2 tsp salt until sizzling, about 3 minutes. Reduce heat to medium-low and cook until garlic is light blond, about 3 minutes. Then pour through fine-mesh strainer into small bowl; press on solids. Measure 1 Tbsp solids and 1 Tbsp of the infused oil into large bowl and set aside. Discard remaining solids, but reserve remaining oil.
Place skewered potatoes in single layer on microwave-safe plate. Poke each potato several times with skewer or fork. Brush with 1 tablespoon of the reserved oil and season with salt. Microwave until potatoes offer slight resistance when pierced, about 8 minutes, flipping halfway through cooking time. Brush with remaining oil; season with salt and pepper to taste.
Preheat gas grill on high heat, then reduce heat to medium. Cook potatoes with lid down for about 4 minutes. Then cook the other side of the potatoes for about about 5-8 minutes longer, until easily pierced.
Remove potatoes from skewers and put them in the large bowl with reserved garlic oil mixture. Add chives (if using) and toss until potatoes are coated. Serve immediately.