Surely I'm not the only one who thinks GP's new E is utterly useless?

Its not useless, free cash if you are fighting him.

But seriously, why does everything cost money to be killed, it makes me not want to use the ability because I don't want to aid my opponent.

It is not. It is not ideal but it gives them room to add power. 1v1 he is now a lane bully who can deal true damage 3 times in 6 seconds and chase anyone down easy. Think about that.

Heres the thing about the new E, in the laning phase if you are up against a ranged champ aka (gnar, an ADC champ that goes top, jayce, etc) then they can just shoot it down no problem. But if you have a melee champ top then you can set up some great things with it. You can use it to scare them out of lane, to set it up to slow any ganking junglers, to clear waves, to set a trap, or just for shits and giggles. Watch the champion spotlight, they show how its affective in lane as well as in teamfights.

Early-mid game aganist ranged it's nothing more than a glorified wave clear under tower.

Late game doing 1k AoE crit ignoring 60% of their armor however...