I main GP and he is probably one of the TOP hidden overpowered champions. He is an extremely complex champion, i'll give you that. Let me start you with your hardest counters:
Riven: Stay away and farm and only farm. She will crush you (assuming he is a capable player: Plat 3+). Use your W ONLY to get out of her Q combo's Stun. Otherwise, you are a goner. Use ult for yourself.
Panth: You are dead. Not a fun lane at all. He is just better. Hug and farm. You'll go oom faster than he will. Use ult for yourself.
Irelia (skill matchup): She is very strong, don't underestimate her, but I haven't lost to one. Do not melee her post 6-13, she will out duel you. However, her damage may surprise you if you face a good one.
That's it for counters.
Now, you want to start your Barrel vs champions like : Darius, Riven, Irelia, Fiora, and Mundo, the rest you start Q.
VS given examples, you will place your barrel between your backline minion wave and frontline. YOUR MINION WAVE, not his/her. Then you will farm and attempt to trade your passive's dmg to test the waters. Once Q is up, you will use it to POKE. Not farm. POKE. Everytime it's up, poke and poke. You will do some devastating damage. Once you have first 1050 gold, you base, fast, buy sheen, and either walk or port back. Your ultimate want to save for bottom lane's fights. This way you will gain EASY assists and kills.
Your ideal Build:
starting item:
first buy:
next buy:
IF FED:
IF regular progression:
Which Pen to use:
Vs Soraka teams AND high heal teams (WW, Swain, etc.) (if two or more || ALWAYS vs Soraka Teams)
Vs Tanky teams (Garen, Chogovich, Tahm, etc.) (if two or more)
Final Build:
Last Item:
(personal fav) ||
Boot Upgrade:
ULT UPGRADE ORDER:

There are many, maaaaaany things to know as GP that I cannot cover in 5 minute read. I could go on for a while.
Couple of tricks I can share:
Prior lvl 7 - Put your barrel in the first bush as safety and escape (since they give you a boost).
When pushing 2nd tier turret, put a barrel infront of the turret and in your back. So if you have to slow them down you can Q the barrel, rush backwards, Then pop the barrel you put behind you for additional speed boost.
Things to keep in mind: Barrels have 40% pen (up to 80% with pen item). Barrels will always crit if Qed. Barrels MELT tanks. Barrels Slow the target.
VS Ranged, start Q and farm with aa.
GOOD LUCK!
I'll make a video sometime. I'll keep you posted.