Drain pineapple, reserving juice; set pineapple aside. In a large bowl, dissolve gelatin in boiling water. Stir in the ginger ale, bananas and crushed pineapple. Transfer to a 13x9-in. dish. Refrigerate until firm.
For topping, combine sugar and flour in a small saucepan. Gradually whisk in reserved pineapple juice. Bring to a boil over medium heat; cook and stir for 2 minutes or until thickened.
Remove from the heat. Stir a small amount into egg; return all to the pan, stirring constantly. Cook and stir until a thermometer reads 160° and mixture is thickened. Remove from the heat; stir in butter. Cool to room temperature.
In a small bowl, beat cream on high speed until stiff peaks form. Gently fold into custard. Spread over gelatin. Refrigerate for 1 hour or until chilled.