Place the sugar, salt, and cornstarch in a small saucepan and whisk until well blended. Add the 7-up and cook over medium heat, stirring occasionally, until mixture begins to thicken. Remove from heat and thoroughly whisk in the lemon juice, and strawberry gelatin. Set aside to cool.
If preparing a pie crust from packaged, preheat the oven to 450 degrees F or according to package directions.
Place the crust into an ungreased glass pie plate and fold the top edges under the pastry (not the pie plate) all the way around. Do this all the way around the top.