Bring the 2 cups of water to a boil.
Sprinkle the lemon gelatin powder over the water and stir until gelatin is dissolved. Add sugar and juice of lemon, stir well.
Transfer the gelatin to a medium bowl and put it, covered, in the refrigerator. Allow to thicken, but not until completely set.
Meanwhile, remove 2 tablespoons of the graham cracker crumbs and set them aside for later use. Add the remaining graham cracker crumbs to a medium-sized bowl.
Melt butter and pour over graham cracker crumbs. Mix well. Spread the crumbs evenly over the bottom of a 9″ x 13″dish or baking pan and press them firmly against the bottom of the dish to create a crust. Set aside.
After gelatin has thickened, beat with an electric hand mixer until fluffy. Set aside.
In a well-chilled bowl, beat cold evaporated milk until thick. Gently fold the whipped evaporated milk into the whipped gelatin. Spread mixture over the graham cracker crumbs. Cover and refrigerate for several hours or overnight.
Just before serving, prepare whipping cream, adding white sugar as needed to sweeten. Spread whipped cream on top of gelatin mixture. Sprinkle with reserved graham cracker crumbs and the lemon zest. Cut into squares to serve.