In a small saucepan, combine the vinegar, sugar, and salt and bring to a boil over medium-high heat, stirring occasionally as the mixture heats. Let the mixture boil for 10 to 12 minutes or until reduced by half.
Reduce the heat to low so that the vinegar simmers. Stir in the thyme, sage, bay leaf, rosemary, and peppercorns and let the gastrique simmer for 3 to 4 minutes. Remove from the heat and set aside to cool for 10 minutes.
Strain the gastrique into a glass bowl and let it cool to room temperature. At this point, the gastrique should be the consistency of light syrup, and thick enough to coat the back of a spoon . Use right away or cover and store at room temperature for up to 10 days.