What is a working way to get all available guis but one (the one you want to open) to be invisible?

You could use an In Pairs loop to close all of the Guis and make visible the one you wanted after, something like this:

local PlayerGui = game.Players.LocalPlayer:WaitForChild("PlayerGui")
local CurrentGui = script.Parent.Parent... --Reference the Gui you want to open using "script.Parent" until the frame is referenced

script.Parent.MouseButton1Click:Connect(function()
    for _, Gui in pairs(PlayerGui:GetDescendants()) do -- This goes through all the instances inside PlayerGui
        if Gui:IsA("Frame") then -- checking if "Gui" is a frame and not something else like a ScreenGui
            Gui.Visible = false
        end
    end

    for _, Frame in pairs(CurrentGui:GetDescendants()) do -- Getting all the instances under the Gui you want to make visible to make sure any frames are toggled
    if Frame:IsA("Frame") then -- Same as the last in pairs loop
        Frame.Visible = true
    end
end)

As a quick note: don't use StarterGui when referencing anything relating to Guis since StarterGui is similar to ReplicatedStorage, it clones all of your Guis into the player's screen, if you want to reference the player's individual Guis use PlayerGui (game.Players.LocalPlayer.PlayerGui) and don't forget to use WaitForChild() to avoid referencing Instances inside PlayerGui that ROBLOX hasn't finished loading yet (WaitForChild("GamepassesBG")).