League of Legends (LoL) Question: Placing barrowls while chasing an enemy

Hmm I do not really think there is any exact strategy for this. I am not a Gangplank player but you can consider going for more movement speed-ish runes like Celerity or Relentless Hunter. Or you can go for more movement speed items like Trinity Force. Remember that Parrrley can instantly trigger Trinity Force and thus gain you movement speed bonus. Maybe try building Boots of Swiftness if you are not in any super hard match up (in patch 10.5 it is suppose to get buffed by +5 MS so it will be even better after that!). Hope I helped at least a bit :/