March 3, 1986
I think she appeared in a photo in the March 3, 1986 strip, as one design choice distinguishing her from Grandma Arbuckle was that she wore glasses in the 1983 appearances.
Grandma Arbuckle prior to 1987.
Aunt Gussie in 1983.
The March 3, 1986 strip. Note the photo.
Addendum - It actually might be Grandma Arbuckle, as Gussie's chin and the bottom of her skull is longer and Gussie's neck is thicker and quite obscured than Grandma's.