Do two monsters spawn on gates when playing with five or more investigators, or is this just a house rule?
Yes, this is an official rule. From the rulebook, p.9/10:
3) A Monster Appears
The first player draws a monster marker from the cup at random and places it on the location. If this would bring the number of monsters over the monster limit, the first player places the monster in the Outskirts instead...
Exception: If there are five or more investigators playing, then draw and place two monsters instead of one.