How would I go about completing this inventory system GUI?

I am making an inventory system and I was wondering how I'd make it automated? Here is what I mean: The slots will automatically be filled when an item is added, if there is already an item it moves to the next slot. If there is no slot, it creates a new one. This is the code I have right now for the frame, I had something else but it was very inefficient. Right now all I have is printing what items the player has. If anyone could either help with the script, point me in a good direction, or tell me what I should learn It'd be very appreciated.

local player = game.Players.LocalPlayer
local b = player.Backpack
local gui = script.Parent
local spots = gui:FindFirstChild("Open1") -- Open Slot
local spot2 = gui:FindFirstChild("Open2") -- Open Slot
local event = game.ReplicatedStorage.InventoryAdd -- Remote event
local salad = game.ReplicatedStorage.f -- Just a remote event for client event
local fol = script.Parent -- The folder where the Slot frames and this script
event:FireServer()  -- this just checks if player has an item and changes the value of the item to true

local pizza = player:WaitForChild("PStats"):WaitForChild("Items"):GetChildren()
for i = 1, #pizza do
    print(i, pizza[i].Name)
    end


local function ItemCheck()
-- Code to go here to update GUI


end

salad.OnClientEvent:connect(ItemCheck)