In a one-cup glass measure, combine 1/4 cup water and orange juice concentrate. Sprinkle gelatin over surface, allow to stand 1 to 2 minutes to soften. Pour juice mixture into blender, add avocado; whirl until smooth. Meanwhile, heat 1 cup water to boiling. Pour into blender, whirl until smooth. Pour into 9 x 9-inch pan. Chill until set. Cut into 1-inch blocks. NOTE: apple juice concentrate may be substituted for orange juice concentrate.