League of Legends (LoL) Question: When should i buy gargoyle? And why?

First, I am sorry for my bad English, it's not my mother language. To be honest, I think Gargoyle is quite a risky item to use especially if you don't have any ability to engage in team fight.
- Gargoyle only give 40 armor and 40 MR, not very high numbers compare to most tanky items.
- Your active gives you a lot of health, but it has cooldown. You rely heavily on Gargoyle's active, if you can't adapt to the game ( bad grammar ), for example: when the enemy team attack, but Gargoyle is on cooldown, it can change the tide of your game, resulting in a lose combat.
- Gargoyle is an active item, it is powerful yet many people forget to active it.
- At late game, Carries can kill you with a few hits, those extra might only protect you from 2-3 attacks.
But if you want to use Gargoyle, I recommend buying as your 3rd or 4th item as the Carries aren't very strong yet and Early and Middle Game usually prefer tanky champions.

You should buy Gargoyle when you need both armor and MR in team fights, as it is 40 armor and 40 MR doesn't worth the cost, but in team fights (3 or more enemies) it double it's worth and it's active is so good.

Buy it mostly when you are the engage tank (such as Leona) as you will be in the middle of the fight and get the most of this item. Remember to use it's active.

You should buy this item after you have at least 1 (big) or 2 (smaller) health items as only resistance won't give you much and the active scale with max health. I mostly buy it as 4th/5th/6th item if needed.