Step 1: In a microwave-safe glass bowl, combine the gum base pellets, corn syrup, and 1/2 tablespoon of glycerin. Microwave in 30-second intervals, stirring vigorously with a silicone spatula after each, until the gum base is completely melted and smooth (1.5-2 minutes total). Be extremely careful, as the mixture will be very hot.
Step 2: Quickly stir in the concentrated candy flavoring, food coloring (if using), and citric acid (if desired). Mix well until the color is uniform and the flavor is fully incorporated into the hot gum base.
Step 3: On a clean silicone mat or a large surface dusted with about 2 cups of powdered sugar, create a well in the center. Pour the hot gum mixture into the well.
Step 4: Using your silicone spatula, begin to fold the powdered sugar into the gum mixture. As it cools and becomes less sticky, switch to kneading with your hands. Continue kneading for 5-10 minutes, incorporating more powdered sugar as needed, until the gum is firm, elastic, and no longer sticky (it should resemble modeling clay). If it feels too stiff, add the remaining 1/2 tablespoon of glycerin, a few drops at a time.
Step 5: Once the desired consistency is reached, roll the gum dough into long ropes, about 1/2 inch thick. Use kitchen shears or a sharp knife to cut the ropes into individual pieces, approximately 1 inch in length.
Step 6: Lightly dust each piece of gum with extra powdered sugar to prevent sticking. Allow the gum pieces to sit at room temperature for at least 30 minutes to firm up before wrapping individually in wax paper or storing in an airtight container.