Sprinkle gelatin over 1/4 cup grape juice in a 1-quart heavy saucepan and let stand 1 minute to soften. Bring to a simmer, stirring until gelatin is dissolved. Remove from heat and stir in remaining 3/4 cup grape juice with lemon juice, grapes, and grappa. Lightly oil ramekins and put in a shallow baking pan. Divide grape mixture among ramekins and chill in freezer until just set, about 30 minutes. Stir together gelatin and 1/4 cup cream in cleaned 1-quart heavy saucepan and let stand 1 minute to soften. Bring to a simmer over moderate heat, stirring until gelatin is dissolved. Add remaining 3/4 cup cream with brown sugar and return to a simmer, stirring until sugar is dissolved. Whisk together yogurt, grappa, and salt until smooth in a large measuring cup, then pour in cream mixture and whisk until combined well. Pour mixture into ramekins (over set gelée) and chill (in refrigerator), covered, until firm, at least 8 hours. To unmold, run a thin sharp knife along edge of each ramekin to loosen, then dip in a small bowl of very warm water 6 seconds. Invert a plate over each ramekin and then invert panna cotta with gelée onto plates, gently lifting off ramekins.