Spider webs appear at the gate mainly because spiders choose to make webs in damp, dark and undisturbed places, which are the conditions at the gate. This has nothing to do with Feng Shui theory, but the ecological behavior of spiders. The appearance of cobwebs can't be sure of good or bad luck, it's just a structure built by spiders for hunting. We should not over-interpret the appearance of cobwebs, but treat this phenomenon rationally.