page 18 of the Arkham Horror rulebook wrote:
MONSTERS GUARDING GATES
During the turn he returns to Arkham from an Other
World, an investigator does not have to evade or fight
any monsters in the gate’s location. This rule only
applies during the turn he returns to Arkham; in subsequent
turns, if the investigator remains in the location,
he must evade or fight any monsters there as normal.