Keep Shadow Ball
Why? Well first off, Liepard is a better special sweeper than Physical. It gets Nasty Plot (Learnt level 50 in XY), so special attacks like Shadow Ball are much better really. Additionally, Shadow Ball generally has better overall coverage than compared to Gunk Shot - Both are super-effective only against 2 types, but Ghost is only resisted by Dark and Normal is immune to it, while Poison is resisted by 4 types.
The only reason you would choose gunk shot would be to take down Fairy-types, which are generally Specially based. Possibly the factor of more power, but remember it takes a large hit in accuracy too. It's the equivalent of Focus Miss.