Gammie Heffley is Greg, Rodrick and Manny's 95 year old paternal great-grandmother and is Grandpa, Arthur Heffley, Reba Heffley, and Loretta Heffley's (and presumably Great-Uncle Bruce's) mother. She is the oldest living member of the Heffley family and the widow of Chester Heffley. She has only appeared in The Ugly Truth, but had a major role in the story. She has a lot of wisdom, and traditionally gives "The Talk" to her older grandchildren and great-grandchildren on the edge of becoming teenagers. Gammie likes visitors as she often makes meals for them. Her furniture is also very valuable, and people often leave notes with their names on her possessions, so they can get it when Gammie dies. Her last appearance in the book is when she gives "The Talk" to Greg about how he shouldn't be too excited about growing up, but Greg leaves after she pulls up her leg to show Greg medical conditions older people can get. In Cabin Fever, it was mentioned that she gave Susan Heffley a spoon carousel.
- Diary of a Wimpy Kid: The Ugly Truth (First appearance)
- Diary of a Wimpy Kid: Cabin Fever (Mentioned)
- Diary of a Wimpy Kid: Class Clown
- Happy Meal Commercial
- The Wimpy Kid: Do it Yourself
- Gammie is one of the few characters in the series to have a confirmed age.
- According to the family tree in "The Wimpy Kid: Do it Yourself", Gammie is on Frank's side of the family.
- In a flashback in Cabin Fever, when Greg snook into his mother’s, Susan, email account to tell everyone to coordinate all of the Christmas presents Greg will get, Gammie’s name can be seen on the computer.
- If the Diary of a Wimpy Kid series was not timeless, by The Long Haul she could be 96.
- It‘s unknown if she’s still alive after the events in Diary of a Wimpy Kid: The Ugly Truth
- Gammie made an appearance in a Happy Meal Commercial created McDonald's along side with The Pig Rodrick,and Sweetie
- In the Long Haul film her age is revealed as 90.