Pack the peeled garlic into clean, sterilized, 8-ounce jars, leaving about 1/2 an inch of headspace. If using, sprinkle in red pepper flakes.
In a small saucepan, combine the white vinegar, sugar, and kosher salt.
Bring to a boil over medium heat, stirring until the sugar and salt dissolve. Remove from the heat, then stir in the pickling spice.
Carefully pour the hot brine over the garlic in the jars, ensuring the garlic is completely covered. Gently tap the jars to remove any air bubbles and adjust to maintain a 1/2-inch headspace.
Let the jars cool at room temperature until no longer warm to the touch, about 1 hour. Seal them tightly, then refrigerate. For the best flavor and texture, let the garlic pickle for at least 3 days before serving. The flavor will continue to develop over 1-2 weeks.