You need both gnatrol and an IPM spray. Gnatrol only takes care of the gnats in the soil. The IPM spray is for the plant above ground.
Here’s the DIY organic IPM spray recipe. Bodhi Testers Summer 2020 - Passionfruit Hasplant (Lush X 88G13/HP) - #57 by nube
Buy the essential oils on Amazon in 2 or 4oz bottles. The neem oil comes in 4 or 8oz bottles usually. Make sure they’re organic. Get the Dr. Bronners at your local supermarket in the health food section. It’s great soap for all around. We wash our dogs, ourselves, and mop our floors with it. The only thing I’ve changed in that recipe is I no longer recommend the optional wettable sulfur. Apparently some brands don’t work with oils.
All those ingredients will cost about $40-60 depending on the quantities you buy, and then it’ll last you for 5+ years if you spray once a week religiously. Follow the instructions on how and when to use and you’ll never see issues.