This is one of the most frequently asked questions, probably I should add the answer to the description... Here is a text from some RPG book:
Gate wards are a bustling part of town, where traders line up to enter the city, sellers hawk their goods, and vendors sell various foods on a stick. Gate wards are second only to market wards in activity. In order to have a gate ward, communities must have gates, or designated areas where people must enter the city. At these areas of entry, some level of inspection, inquiry, or taxation of merchants usually takes place.