How do I make a GUI turn off after leaving a seat?

A better way to do this is to listen for the "Occupant" property of the seat being changed. Here's how it should look:

local seat = workspace.Seat

function SeatChanged(propertyChanged)
    if propertyChanged == "Occupant" then
        if seat.Occupant then
            game.Players.LocalPlayer.PlayerGui.seatgui.Frame.Visible = true
        else
            game.Players.LocalPlayer.PlayerGui.seatgui.Frame.Visible = false
        end
    end
end
seat.Changed:connect(SeatChanged)

What this code does is connect the SeatChanged function with any property of the seat being changed. From there, it checks if the property that was changed was "Occupant", which is the player when the seat is occupied. Then, it simply checks if the player exists in that property; If not, it closes the GUI, if so, it opens it.

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