Most of the time, Gertie acts like she's crazy, usually having a different theme in her clothes, preparing different foods, calling people other names or labels (Arnold is "Kimba", and Helga is "Eleanor Roosevelt"), and mixing up holidays. She is very upbeat, and takes any situation with the same enthusiasm as any other. While kind of senile and a bit crazy, Grandma is an enthusiastic, supportive, and loving grandmother. She loves Arnold very dearly and will often be very protective of him.She often is used as a philosophical source, especially with Arnold, through the use of metaphors that are usually not understood.
Her zany and eccentric behavior has been attributed to senility, though it's also implied to be on some level intentional (possibly for her own amusement), as she drops the act completely during more serious moments.
While Phil is physically strong for his age, Gertie is far more active and capable than him and almost anyone else in the show. Besides being a black belt in martial arts, she can also take on larger conflicts such as repairing the underground subway's wiring, hijacking a bulldozer, or escaping from prison.
Grandma Gertie (or otherwise known as "Pookie" by Phil) is Arnold's paternal grandmother. She is the wife of Grandpa Phil, and they have known each other since childhood. They had a relationship like Arnold and Helga when they were young. Gertie was mean to Phil (although in more extreme ways, like putting thumbtacks in his seat, dunking a part of her hair in ink and blaming Phil for it, etc.). Formerly a librarian and a police officer, Gertrude now runs a boarding house with Phil, and they are the parents of Arnold's dad, Miles.
Examples of Grandma Gertie's wacky outfits, costumes and acts include the following:
- In "Eugene's Pet", Grandma dresses as a mermaid and serves fish.
- In "Arnold's Thanksgiving", Grandma gets the entire family to dress as important figures in American history in celebration of the Fourth of July. It's noted that she celebrates Thanksgiving on the Fourth of July. However, in the end, it ends up a normal Thanksgiving (with fireworks).
- In "Parents Day", Grandma wears her karate outfit throughout most of the episode, and is cooking miso soup.
- In "Summer Love", she sees a nudist beach. She then takes off her suit to join the nudists, scaring them away.
- In "Family Man", she dresses as Mary, Queen of Scots. While Mr. Hyunh is pretending the boarders are his immediate family, she pretends she's also Mr. Hyunh's sister.
- In "Rhonda Goes Broke", she dresses up as a Spanish Conquistador and gets on top of the table saying "I claim this dining room in the name of Spain" while every one in the boarding house is having dinner.
- In "April Fool's Day", on April 1st, Gertie runs into the house screaming "Happy Groundhogs day I saw my shadow which means..." and Arnold and Phil continuing non-too ecstatic saying "12 more days of Christmas". She puts up Christmas decorations and takes Arnold to the dance in a sleigh.
- In "Save the Tree", Grandma wears a Hippy outfit and a Tarzan suit.
- In "Girl Trouble", she wears a kimono.
- In "24 Hours to Live", "Gerald Comes Over", and "Grandpa's Packard", she dresses in a safari outfit and calls Arnold "Kimba".
- In "Friday the 13th", she dresses as a black cat.
- In "Grandpa's Packard", she dresses as a police detective from the 1950's, calling herself "Spade."