Supportplank doesn't work because:
- His harass runs him out of mana after a few Qs. On top of that, the gold increase from your Q isn't even going to get you much extra.
- Unless you're running targons, your adc will not appreciate you taking last hits for your Q.
- His ult sucks unless you build AP, in which case, the only other good spell you have is oranges, and maybe lich bane on Q
- The only thing you peel is oranges. The piddly slow on his passive doesn't get you shit, and you ult isn't up often enough to be worth using just for that.
- Most supports offer some form of engage or disengage. GP offers a minor ms buff and a global ult with a slow and shitty damage. While that can be, hypothetically speaking, a form of either, it's extremely subpar for both.
- No hard cc.
- No heals.
- Late game damage potential is also covered by most viable mage supports, and better. Annie and Brand come with hard cc, and better damage. They don't have to choose between builds, either.
- Minimal escapability.
Basically, all he really adds to a lane is his poke and minimalsoft cc. Other supports that have poke can usually spam it more, and come with more utility on top of that. The reason this works in silver is because a lot of silver supports are there for a reason. Many can't ward for shit, don't know how to apply pressure, can't help set up last hits under tower, aren't proficient on their champ, don't know when to push or freeze, don't know when to go for kills or back off, or just generally suck. If they think supporting is standing there soaking xp and not doing anything until you get engaged on, of course you win on GP.
The thing is, you'll see all variety of shit like that in silver and especially bronze. So yes, fucking anything works in silver. Just because GP has some poke and soft cc doesn't make him viable as support. By that logic, anyone can support. Honestly, if you want an unconventional support, pick ashe. Longer range poke, better hard engage (if well aimed and timed, can be 4 seconds (also read: a free kill)), more (fuck tons) slow, crit potential in lane, long range vision ability.