Is This For A LocalScript Or Server Script?

Mouse = game.Players.LocalPlayer:GetMouse()

Mouse.KeyDown:connect(function (key)
	if key == "G" then
		Part = Instance.new("Part", game.Workspace)
		Part.Position = game.Players.LocalPlayer.Character.Torso
	end
end)

You're trying to get the LocalPlayer's mouse which you can only do in a LocalScript.

So it is a LocalScript.