Don't listen to the 2 dumb comments prior.
In the case of GP vs Fiora you play GP as a farm lane and play safe.
Fiora's counter prevents your barrels and Q, so if you want to attempt to fight fiora you'd need a jungler.
GP can outscale her if you call your mias and farm until mid game and use your ulti mainly to aid mid/bot because fiora again counters most of your kit.
If you really want to fight her however place your barrels but don't Q them, just harass her with Q and if trying to damage her with barrels use AA's since then if she counters that you have Q to hit her with again but she will win the trade the longer you take to run away afterwards.
Optimally if she all-ins you want to AA her->Q her->AA barrel if she stays->AA her again and run the hell away hoping you outdid her damage. She will counter some part of this you have been warned.
While if you want free harass you want to barrel and AA them to hit her and if she counters it you Q her or mostly harass her with Q and if she counters one you AA a barrel in her face.
Either way to trade with fiora be ready to give up some Q mana.