In small bowl, combine graham cracker crumbs and melted butter until thoroughly moistened. Press onto bottom and up sides of 11 inch tart pan. Refrigerate while making filling.
For filling, whisk grapefruit juice and cornstarch in medium saucepan until dissolved. Add sugar, egg yolks, zest and salt; whisk until well combined. Cook, stirring constantly, over medium-low heat until mixture thickens and boils, about 7-8 minutes. Reduce heat to low and continue cooking and stirring for 30 additional seconds. Remove from heat and stir in butter until melted and completely mixed in. Spread evenly over crust. Place plastic wrap directly on filling; cool completely.
In large bowl, whip meringue powder, water, and half of sugar with electric mixer at high speed for 5 minutes. Gradually add remaining sugar and continue whipping at high speed 5 minutes or until meringue is stiff and dry.
Remove plastic wrap and pipe or carefully spread meringue over top of tart. Refrigerate until ready to serve.