GUI Grid + Padding Math and Generator

local GUI = Game:GetService("Selection"):Get()[1]

if (not GUI or GUI.Name ~= "ItemContainer") then
    error("Invalid inventory GUI selected")
end

local AbsoluteWindowSizeX = (GUI.AbsoluteWindowSize.X-20); -- We take away 20 for 10 pixel padding on both left and right sides
local AmountOfGUIsPerRow = 3;
local TotalGUIs = 30;
local TotalRows = math.floor( (TotalGUIs/AmountOfGUIsPerRow) + 0.05)
-- Here's an interesting part.
-- Depending on how many GUIs in each row we have, we need an amount of spaces in between them equal to AmountOfGUIsPerRow-1.
-- Then we need to determine how many pixels in between each one and also subtract that from the AbsoluteWindowSizeX
local PaddingInBetweenGUIs = 10;
local AbsoluteWindowSizeX = AbsoluteWindowSizeX - (PaddingInBetweenGUIs * (AmountOfGUIsPerRow-1));
local SizeOfEachGUI = math.floor( (AbsoluteWindowSizeX/AmountOfGUIsPerRow) + 0.05);

-- Should be it. Generate the GUIs in the selected container.
for Row = 1, TotalRows do
    for Item = 1, AmountOfGUIsPerRow do
        local ShouldPadX, ShouldPadY = false, false;
        local ImageGUI = Instance.new("ImageLabel");
        ImageGUI.Size = UDim2.new(0,SizeOfEachGUI,0,SizeOfEachGUI);
        ImageGUI.BackgroundTransparency = 1;
        ImageGUI.Image = "";
        ImageGUI.ZIndex = 2

        local FallbackText = Instance.new("TextLabel");
        FallbackText.Size = UDim2.new(1,0,1,0);
        FallbackText.BackgroundColor3 = Color3.new(30/255,30/255,30/255);
        FallbackText.TextColor3 = BrickColor.White().Color;
        FallbackText.Text = "Inventory Item Name"
        FallbackText.TextWrapped = true;

        -- Do not pad first or last GUIs
        if (Row > 1) then
            ShouldPadY = true;
        end

        if (Item > 1) then
            ShouldPadX = true;
        end

        ImageGUI.Position = UDim2.new(0,10 + ((SizeOfEachGUI + (ShouldPadX and PaddingInBetweenGUIs or 0)) * (Item-1)),0, 10 + ((SizeOfEachGUI + (ShouldPadY and PaddingInBetweenGUIs or 0)) * (Row-1)))
        ImageGUI.Name = "[" .. tostring(Row) .. "," .. tostring(Item) .. "]"
        ImageGUI.Parent = GUI
        FallbackText.Parent = ImageGUI
    end
end