'No cap' and 'Gaslight' are Gen Z slang. 'No cap' means 'truthfully' and 'Gaslight' is a metaphor for manipulating someone's thoughts.
The dad is putting his kids on with a mechanic because he thinks the slang is about his car. (He mistook 'No cap' for there being a cap missing on the car, and 'Gaslight' for the light that appears when the car's low on gas.)
