Hi, I'm Achiwa.
I was Silver 3 last season, and climbed up there from B1 with Gangplank and Twitch.
I've tried several GP builds, and I think this one is probably the best for solo queue climbing.

Yeah I know, I'm not the best at the game, by far, but my builds have helped me support my team and bring home the win.

I use standard straight armor for most of my runes, as we want to farm into latergame and pretty much just be unkillable providing some utility to our team. Scaling MR helps with the enemy mid for later, assuming you aren't against an AP top lane.
AD quints just help your early game for trading and farming.

9/18/3 page with the AD and CDR in offense, general tankiness in defense, and mana regen in utility.

Winning/even? Max Q.
Getting owned? Max W.
Teamfighting a lot early? Max E.

I would personally never pick GP against Pantheon, Rumble, or Olaf.
Panth will block your Q and just outtrade you straight up.
Rumble scales even harder than you once he gets some magic pen.
Olaf.. True damage on a 7 second cd is insane.

GP is pretty good against virtually anyone who has no natural sustain or has to rely on all-ins.
So pretty much Darius , Wukong , and maybe Akali .

Pretty much, If you need a tanky bruiser that still deals a lot of base damage, Tankplank is the one for you.