Grasp of the Undying is a top tier pick on GP. One of the best ways to get a good laning phase with GP. Really there isn't any Rune that super focuses on your Barrels so it's better to focus on your Q and early trading, in my opinion. Grasp chunks the hell out of people early game and because you can spam it, it gets super juicy for early trading.
Klepto GP is kind of meh honestly?
Comet is a possible opinion but I don't think it's very strong.
You could try the Freeze one for sticking power but it's just kind of pointless on GP. I would go with Klepto or Grasp in the long run. Klepto boosts your late game by getting you there sooner while Grasp boosts your early and mid by giving you a laning phase.