Want a position virtually nobody plays?
Want to get your team fed by shutting down enemies with your massive CC and make them waste all of their skills on you while you tank their attacks?
WANT TO BE A PIRATE?
Play support gangplank.

Support Gangplank is very flexible, after the 2 core support tank items (face of the mountain and sightstone) you can basically build anything you want. I highly recommend building a frozen mallet somewhere in there over any other item simply because of it's amazing potential to set up kills and kite, remember your Q applies melee on hit effects, getting maximum CC from the mallet.

IMPORTANT: BUY MANA POTIONS EARLY GAME IN PLACE OF HEALTH POTIONS, PRACTICE BEING ABLE TO STAY ON LANE WITHOUT HEALTH POTIONS, THIS IS WHAT WILL MAKE YOU A GOOD SUPPORT GANGPLANK IS BEING ABLE TO USE YOUR ABILITIES AS OFTEN AS YOU CAN, YOU CANNOT PASSIVELY PLAY SUPPORT GANGPLANK AND GANGPLANK GETS VERY MANA STARVED.

AP items, especially liandary's torment can help you with your ult, making sure the enemy takes plenty of damage for your team to clean up.

Zeke's Herald is something I almost always build as well, it works especially well with an auto attack heavy team.

Righteous Glory is a great initiation item, combine it with raise moral and your team can outpace the enemy in movement speed no problem.

Randuins is also a great CC and tank items!

I will glance over some general situational item info: Mana items can help gangplanks who want to eat biscuits instead of drink mana pots. Less supporty tank items can be used if one needs to do a bit more carrying and less supporting. Hydra specifically should be taken if your entire team is lacking good waveclear.

Make sure to max out Q, then max out E, then max out W. In case the enemy is much stronger than you in laning phase, get W interchangingly with Q.

Support gangplank is good at taking down more farm oriented lane, or ones with weak mid games. Gangplank also heavily specializes in being able to essentially gank other lanes using his ult as a slow and damaging set up for allies to jump in for the kill. Gangplank can empower entire teams during fights and command the tides of battle to his favor. Come teamfight phase there are very few weaknesses to be found in gangplank, he can kite, damage, tank, sustain, and initiate. Gangplank's biggest problem is laning against any support who can outrange him or outdamage him whenever gangplank tries to poke with his parrrley. Also he forces the player to grab clarity and mana pots with the amount of spell spam he requires.