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."
- In Field Trip, when Gertie and Arnold drive back to the aquarium in Phil's Packard to rescue Lockjaw, Arnold worriedly asks his Grandma if she has a driver's license. Gertie responds by laughing maniacally and putting the pedal to the floor. This implies that Gertie either had to give up her driver's license at some point due to medical issues or never had one to begin with, and that she more than likely obtained the keys to the Packard without her husband's knowledge. In Grandpa's Birthday, when Phil is writing out his will, he forces Arnold to take the Packard. When Arnold asks what he is going to do with a car, and why he can't give it to Grandma, Phil responds by saying "Are you crazy? Would you trust your Grandma behind the wheel of a car?!?! Now, I'm leaving you the Packard, okay?!?!" In Grandpa's Packard, Gertie rides a motorcycle, with her grandson in the sidecar, for much of the episode. Arnold expresses no concern about his grandmother's driving ability, or whether or not she has a valid license to drive a car or ride a motorcycle. Gertie more than likely does have a driver's license, but probably wasn't a very good driver to begin with, and her old age has exacerbated the problem.
- Gertie may have been a police officer in the City of Hillwood at one point.
- Gertie and Phil knew each other since grade school, but did not go on their first date until they were adults.