Linda Duncan is the grandmother of PJ, Teddy, Gabe, Charlie, and Toby. She is the mother of Bob and former love-interest wife of Frank. She made her first real appearance in Welcome Home and a gag appearance in Dress Mess.
Her real name is Linda, rather than Gladys as said in "Dress Mess", because in "Welcome Home", she makes her first debut appearance and is referred to as Linda.
She and Frank split up prior to "It's a Charlie Duncan Thanksgiving".
In "Dress Mess", she was portrayed by Eric Allan Kramer but in "Welcome Home" she is portrayed by Shirley Jones, probably because the events that happen during the credits of a Disney Channel episode usually are intended to be jokes and do not count as real information to be learned about the characters.
It is possible that Linda's last name is not Duncan as she divorced her husband, Frank Duncan, and may have changed back to her maiden name which is currently unknown.
She has a bit of a habit with using the words "what not" way too much to the annoyance of her daughter-in-law Amy.
In "Dress Mess", in the ending credits, Linda was portrayed by Eric Allan Kramer but was named Gladys, and was meant to look exactly like Bob. She asked the Doctor what gender her new grandchild will be. She looks like Bob but has gray hair, wears pink and carries a handbag with glasses. The doctor assumed she was Bob in disguise but then Bob came out with Gladys. These ending credits were just a joke and were not meant to be true.