In bowl pour 1/2 cup cold water. Sprinkle the unflavored gelatin over to soften, about 10 minutes.
In a non-reactive saucepan, combine 1 cup water, the vinegar, sugar, and salt. Cook and stir over medium heat until hot and sugar is dissolved. Remove from heat; add softened gelatin mixture, stirring until dissolved. Transfer to large bowl.
Place bowl over very large bowl of ice water. Stir occasionally until completely cold and just beginning to thicken, about 10 minutes.
Stir cucumbers, green onion, and celery into cold mixture. Pour into a 6- to 8-cup non-reactive mold or dish that has been lightly oiled. Wrap mold with plastic wrap; refrigerate 4 hours or overnight.
To unmold, dip mold briefly (10 seconds or less) into very hot water. Invert mold onto a serving dish. Makes 8 servings.