The ornamental design for a gallonjuginsulator, as shown and described.
FIG. 1 is a perspective view of a gallon jug insulator, shown in use with a gallon jug and shown with a wallet held within a pocket of the gallon jug insulator; the gallon jug and wallet are shown in broken lines and form no part of the claimed design;
FIG. 2 is a front view of the gallon jug insulator of FIG. 1 shown with a cellular phone held within another pocket of the gallon jug insulator and shown with keys coupled to the gallon jug insulator; the cellular phone and keys are shown in broken lines and form no part of the claimed design;
FIG. 3 is a rear view of the gallon jug insulator of FIG. 1;
FIG. 4 is a right side view of the gallon jug insulator of FIG. 1;
FIG. 5 is left side view of the gallon jug insulator of FIG. 1;
FIG. 6 is a top view of the gallon jug insulator of FIG. 1; and,
FIG. 7 is a bottom view of the gallon jug insulator of FIG. 1.