General Playstyle

IMO Greystone performs much better with a split push playstyle as opposed to a heavy grouping and teamfight playstyle (though you should still be grouping with your team when convenient). There are better tanks for teamfighting. However, Greystone excels at dueling, escaping, and split pushing. His primary ability enables him to clear waves very efficiently. His secondary, alternate, and ultimate enable him to survive and escape ganks while drawing the enemies resources for an extended period of time. What he does struggle with is sticking to enemies. Outside of his secondary ability, he has no means of locking down enemies. This build aims to increase his ability to lock down enemies during fights and improve his dueling abilies.

Early Game

To preface, I think the following guide on wave management will help you get to Platinum in this game extremely easy as an offlaner if you can master these fundamentals: https://www.youtube.com/watch?v=4_LjLmvBWTg
MOBAs are snowbally games so securing an early lead lets you snowball and become a monster in the mid to late game; whereas, playing the early game poorly can make it 10x harder to win the game as you will be playing catchup during the mid and late game.
In the early game you first objective should be to scout where the enemy jungler is starting in order to determine if you will be weak-sided or strong-sided early. Based on that information we can determine how we want to play the first few waves.

Avoid Early Gank

If you are weak-sided, your goal should be to have the wave near your tower at around 2:30. In order to do this, you can either opt to try for a 2-wave crash if the matchup is favorable for you or just concede priority and play safe.

2-wave crash

In order to perform the 2-wave crash you want to build a 1-2 minion advantage on the first wave, and then fast push the second wave and get it to crash under tower as the opponents 3rd wave approaches the tower. This will cause their minions to begin slow pushing towards your tower where you can potentially then move into a freeze. This set up should have the second wave crash a bit before the enemy jungler finishes their first 3 camps which will allow you to play safe at no risk of a gank

Conceding priority

The goal here is to let the enemy push the first few waves into your tower. You'll want to make sure not to miss any exp. It is okay to miss a couple of last-hits in order to preserve HP.

Set Up a Gank

If our jungler is starting on the opposite side of the map as us, then we can try to set up a gank by performing a 3-wave or 4-wave crash which will cause the enemy wave to slow push into us when our jungler is on our side of the map. From there the enemy will be forced to walk up for last hits making for an easy gank.

Item Justification

Slow from Iceskorn Talons and Bonesaw help Greystone stick to targets which he can struggle with. Also, since he spends most of his time at low HP during fights thanks to his ult, Onixian Quiver and Salvation perform really well. Attack speed from Earthshaker, Bonesaw, and Onixian quiver enable him to take towers really quickly since you can very quickly use your empowered E attacks. His primary ability lets him stack Basilisk and Earthshaker really quickly. Go Citadel next for teamfighting. Salvation can be swapped out for Tainted item instead if needed.
Q > E > RMB

Disclaimer

My games often end right after I hit 4 items, so I'm not really sure what the best last two items are. Citadel and Salvation are just the two items I plan on testing.