Combine cantaloupe, sugar substitute, and lemon juice in container of an electric blender or food processor; cover and process until smooth. Transfer mixture to a medium bowl.
Sprinkle gelatin over cold water in a small saucepan; let stand 1 minute. Cook over low heat, stirring until gelatin dissolves, about 4 minutes. Add to cantaloupe mixture, stirring well. Add yogurt, stirring until smooth.
Pour mixture into an 8-inch square pan; freeze until almost firm. Transfer mixture to a large bowl; beat at high speed of an electric mixer until fluffy. Spoon mixture back into pan; freeze until firm. Scoop into individual serving dishes to serve. Garnish each serving with a cantaloupe wedge, if desired.