Wash your grapes thoroughly under cold water and pat them completely dry with paper towels. Line your baking sheet with parchment paper and pour the flavored gelatin powder into a shallow bowl.

Add the sugar, water, corn syrup, and cream of tartar to your medium saucepan and stir gently to combine. Place the pan over medium-high heat and stir just until the sugar dissolves completely, about 2 minutes.

Once the sugar dissolves, stop stirring completely and let the mixture come to a boil. Clip your candy thermometer to the side of the pan and heat until it reaches exactly 300°F (hard crack stage), which usually takes 8 to 10 minutes.

Once the syrup reaches 300°F, immediately remove the pan from heat and let it sit undisturbed for about 2 minutes to allow the large bubbles to subside.

Pierce each grape with a wooden skewer or toothpick, then dip it directly into the hot sugar syrup. Rotate the grape gently to ensure complete coverage, then lift it out and let excess syrup drip back into the pan for a few seconds.

Immediately after dipping each grape in the sugar syrup, roll it in the flavored gelatin powder while the coating is still wet, making sure to coat the entire surface evenly.

Carefully place each coated grape on your parchment-lined baking sheet, leaving space between them so they don't touch. Remove the skewer gently by twisting it slightly as you pull it out.

Let the candy grapes sit at room temperature for at least 2 hours to allow the coating to harden completely. The surface should feel hard and dry to the touch rather than tacky.