No, it will cause a massive black hole which will consume the universe!
I assume you are referring to the fact that the GK cabinet is 400 watts @ 8 ohms and the LH500 is a 500 watt head? It's 500 watts @ 4 ohms so it is not going to put out 500 watts with your single 8 ohm cabinet. Even if it did you could still use it as long as you didn't turn it up to full volume. I'm currently running a 750 watt @ 4 ohm head into a 4 ohm cabinet rated at 600 watts with no problems.
