How do I make loot spawn in my custom GUI?(I think I posted it in the wrong channel)

So I posted this in a wrong forum section last time I think. I don't know these forum choices are really confusing, but basically I made a mob that has GUI and I want to make items spawn in his GUI, but I really can't do it. There is simply no trigger option for opening the GUI for the first time or spawning the items right away, the only triggers that are there(input slot) are for item change. I have looked at tutorials, forum posts, but I just can't figure out.