Ok so I have an entity and whenever it hits another player I want it to show the player that created the entity to see, but not interact with, the hit player's inventory. I have it set up so the entity onImpact calls MainClass.proxy.method() and in that method in my client proxy I open a guiScreen for the players inventory. However, when I am on a server with another user, whenever the entity hits the other player, it calls the commonProxy method. How would I make this work?

I assume youd need packets to update the players client and posting code and what version you are on helps solve problems a lot faster

If inventory - No, you need normal guis with containers... Just look in tutorial of guis and containers, and instead of creating more inventories use player's... Or call player.openInventory...

If simple gui without slots - check that code is run on both sides...

And code would be helpfull...

Where do I put the GuiContainer and Container?