Opening the hatch and pouring in the gelatin as well as the oxygen in the water could have caused the oxidation.
You could try boiling the water, letting it cool, mix the gelatin, and add it to the keg via the in post. You’ll need some sort of syringe attached to a quick disconnect to push it in.
Or, you could just let the keg sit cold until it clears on its own. Choosing a highly flocculant yeast helps.
Gelatin works to a point but there’s no substitute for time IMO. The beer seems to be brighter given time vs gelatin. But I have had those beers that just wouldn’t clear so I hit them with fining agents.