Plot hole: Gary is still "alive" because his head/brain was not destroyed when Shiela killed him. However, over the course of the show, we see many bodies in the freezer or otherwise with the heads still intact yet none of them are still "alive" in spite of the fact that their brains have not been dispatched.
Santa Clarita Diet (2017)
1 plot hole in show generally - chronological order
Knighttime - S3-E2
Other mistake: When Abby uses her phone to show Eric the pictures that Sven posted on Instagram, her phone states it is 11:26pm when you can see it is daylight after they walk back into Eric's room straight after. (00:05:00)
Joel Hammond: Okay, we say we came across this murder site and we're just cleaning it up.
Sheila Hammond: Who cleans up murder sites?
Joel: I don't know. We're Mormons?
Sheila: Mormons don't clean up murder sites.
Joel: Mormons are helpful.
Trivia: At the comic book store, when Eric and Abby are talking, there is a "Plants vs Zombie" comic on the rack in front of them.