Different people have different ideas of what gooning means, but for the most part it's the act of being in an intense, almost hypnotized state of mind after spending hours edging / chronically masturbating.

For instance, you "gooning" over her would be you not being able to think about anything else but your dick while masturbating to her for an hour without cumming because you're so obsessed with how your dick feels.

Many clip sites ban the words "hypnotize" and "poopers" so sometimes "gooning" and "goon juice" gets used as a replacement.