I am trying to close my custom inventory when I press a key but the the entire KeyInputEvent method doesn't seem to be called at all when I am inside my GUI. I can open my GUI just fine using the same method but once the GUI is open the method is called.

 

public class KeyHandler 
{
@SubscribeEvent
public void onKeyInput(KeyInputEvent event) 
{
	if(!FMLClientHandler.instance().isGUIOpen(GuiChat.class)) 
	{
		if(KeyBindings.openInventory.isPressed()) 
		{
			References.network.sendToServer(new OpenPlayerInventory());
			References.network.sendToServer(new ClosePlayerInventory());
		}
	}
}
}

It is possible, just not with the KeyInputEvent - make a public static reference to your KeyBinding somewhere, and then override the #keyTyped method in your GUI class:

@Override
protected void keyTyped(char c, int keyCode) {
super.keyTyped(c, keyCode);
if (keyCode == KeyBindings.yourKey.getKeyCode()) {
	mc.thePlayer.closeScreen();
}
}

That only works for your own Gui though.

Yes, but that's what the OP said he was trying to do:

I am trying to close my custom inventory when I press a key