Dissolve lemon gelatin in boiling water in a bowl; stir in cold water. Chill in refrigerator until partially thickened, 30 to 40 minutes.
Chop cranberries in a food processor and mix sugar into cranberries, pulsing once or twice to combine. Let cranberry mixture stand a few minutes to let sugar dissolve. Stir cranberry mixture, pineapple, celery, and walnuts into gelatin. Pour into a gelatin mold or serving dish. Refrigerate until fully set, about 4 hours.