In a mixing bowl, soak the gelatin in warm water until it softens and dissolves, stirring occasionally. Gradually stir the lime juice and zest into the gelatin and set mixture aside.
In a large bowl, beat eggs, sugar, and salt at high speed until thick and foamy, 7-10 minutes. Reduce speed to low and beat in key lime mixture.
Cover bowl and refrigerate until mixture begins to set but is still liquidy- approx 1 hour.
Whip 1 cup of whipping cream until it forms soft peaks and fold into key lime mixture.
Pour half the key lime mixture into a large bowl and sprinkle a layer of crushed graham cracker crumbs on top, reserving 3 tablespoons. Fill bowl with remaining key lime mixture and refrigerate for at least 2 hours or overnight.
Before serving, whip remaining cream with confectioners' sugar and vanilla extract until soft peaks form. Pipe or spoon cream on mousse and sprinkle remaining graham cracker crumbs on top.