I play in predominately North America. I have an account in EUW, but I rarely use it.
The following are my accounts:
My main was permabanned. If someone hasn't taken the name yet, it is called "Trichiliocosm".Currently, my main account is "GentleCalm".

http://na.op.gg/summoner/?userName=gentlecalm
http://na.op.gg/summoner/userName=trichiliocosm
https://na.op.gg/summoner/userName=captivebolt
https://na.op.gg/summoner/userName=gangplanksupport

I have played League of Legends since Season ~5, but I have forgotten the account I played on at that time. I generally only play offmeta support, but I am capable of playing top and jungle.

Offmeta supports I play with decreasing skill level:
Gangplank > Lee sin > Riven > Zed

Pros:
-poke
-free "q" procs
-global ult slow/dmg
-lots of damage later on
-you may be focused down instead of your ADC because of your damage, therefore peel
-is capable of clearing waves in order to deny xp & teamfight
-capable taking towers down fairly easily with adequate farm

Cons:
-may have difficulties with hard engage supports that can't be killed easily (Leona, Alistar, etcetera), however can be cleansed with your "w"
-may have troubles with assassins
-requires fairly skilled Gangplank player
-in my experience, does not play with weak ADC champions early game (Vayne), but may not matter as much for different players

Flash
-always take flash

Ignite
-if Gangplank player believes that winning early game is essential or is extremely confident in getting kills

Exhaust
-if playing against assassins, but then you do not have as much splitpush/tower taking potential

Teleport
-because you are taking "Demolish" in "Resolve" tree in conjunction with double ignite, and so capable of taking down towers fairly quickly (however it is dependent on your farm compared to tower armor which is correlated to game time I believe, maybe health I'm not sure)

If you need to read this, you should not be playing Gangplank support.

R>Q>E>W in all cases

Image of Runes

Kleptomancy: Q gold in lane in conjunction with bandit from relic shield

Perfect timing: If being dove under tower, or situations where Perfect timing becomes necessary

Magical footwear: Skimping in gold to get trinity force, so boots gives increased movement speed

Celestial body: Can be swapped if more suitable rune is found, I prefer it for increased survivability and my general role early game/in lane is for poke (-10 dmg mitigation doesn't matter as much if poke, but may have secured kill against hard engage lane)

Resolve: +20% potion and elixir duration, +145 health

Demolish: For pushing when time is limited

Second wind: For sustain in lane if difficulties arise

Starting:
-relic shield (hp, sustain, helps ADC)
-refillable potion (less additional hp regen at first, but more efficient overall)

Required, but order of purchase is variable (boots are not necessary because of runes and should not be upgraded)
-trinity force (must be bought ASAP)
-static shiv (AA speed, aoe damage, waveclear, crit, movement speed, crit)
-phantom dancer (dmg mitigation, attack speed, movement speed)
-infinity edge (+AD, +crit damage, +crit chance)

Additional items:
-death's dance (gives lifesteal from "q"ing barrels, prevents from being 1 shot against assassins)
-duskblade of draktharr (lethality with a little more damage due to unique proc)
-edge of night (same as youmuu's except against more of an AP composition)
-lord dominik's regards (against high hp champions with armor)
-youmuu's ghostblade (early powerspike, movement speed)
-mortal reminder (if playing against heavy lifesteal or spell vamp, and they also have high armor)
-sterak's gage (if heavy assassins)
-maw of malmortius (buy if enemy have heavy ap)

1.) Squishy killer
Relic shield + refillable potion > sheen > phage/stinger > phage/stinger > trinity force
> Youmuu's ghostblade > duskblade of draktharr > infinity edge > phantom dancer/static shiv
> phantom dancer/static shiv

2.) Sustain
Relic shield + refillable potion > sheen > phage/stinger > phage/stinger > trinity force
> vamnpiric scepter > executioners calling > infinity edge > static shiv > phantom dancer > death's dance/mortal reminder > death's dance/mortal reminder

Should be VERY careful before level 3 due to hard engage supports
Should acknowledge how strong their engage is and their poke
Should acknowledge what their stuns are and how to outplay their champions as if 1v1 in mid/top
Try to prevent minions from hitting tower
If their wave is too large, you can soften the wave with barrels
Can auto or q to proc relic shield
If 1v2 bot lane for whatever reason (your adc died), and they are pushing, clear wave with ult or barrels
At level 6 be cognizant of the map and use ult/R when suitable
Place barrels in the bush or in your minion wave in order to harass or give lane pressure when their adc goes to last hit or support goes to proc relic shield/pick up coins from ancient coin
If playing against hard engage support that targets you, you can either choose to "w" the stun or save it for increased healing if you are dealt more damage later on

Trading
-Q
-triple barrel combo
-double barrel melee with q

You can clear waves that are in danger of taking your turret
You can push for whatever reason in order to deny xp or take objectives, etcetera
Generally play as typical Gangplank

It is almost mandatory you go with the trinity edge > lethality > lethality because it has the best midgame presence, anything else would be generally for lategame or to be built for bruiser, the sustain build i put up has a long build up time and the damage relative to gold spent will not be as much as the lethality at least for mid game, maybe not even as much damage later at least for squishies, sustain seems to be only for something like Swain, but even then may be unnecessary