If using gelatin sheets, soak them in a bowl of cold water for about 10 minutes, then squeeze out the excess water. If using powdered gelatin, mix it with the tangerine juice and let sit for 5 minutes. Heat the tangerine juice, zest and sugar in a saucepan, stirring, until the mixture is warm and the sugar is dissolved. Remove from the heat and dissolve the gelatin in the juice mixture.
Pour into serving glasses or bowls. Refrigerate until set, about 4 hours. Serve with whipped cream and cookies.