How do you make a code that opens a GUI when stepping on a part but closing it when you step off it?

There's multiple ways to go about this. I'm going to assume you're using a localscript, because you're referencing script.Parent, which is typically a GUI.

The best method would likely be a client event, which is basically an event that you can fire from the server to the client.

It's defined typically in localscripts.

game.ReplicatedStorage.gui_appear.OnClientEvent:Connect(function(vis)
	local appearing_frame = script.Parent -- Just reference the frame you want to appear.
	appearing_frame.Visible = vis
end)

On the server, you'd want to input the .Touched events, because it can only fire on the server.

workspace.openPart.Touched:Connect(function(hit)
	local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
	game.ReplicatedStorage.gui_appear:FireClient(plr,true)
end)


workspace.closePart1.Touched:Connect(function(hit)
	local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
	game.ReplicatedStorage.gui_appear:FireClient(plr,false)
end)


workspace.closePart2.Touched:Connect(function(hit)
	local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
	game.ReplicatedStorage.gui_appear:FireClient(plr,false)
end)


workspace.closePart3.Touched:Connect(function(hit)
	local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
	game.ReplicatedStorage.gui_appear:FireClient(plr,false)
end)

There is likely a better way to go about this, but this is the simplest.