Leaving town through the City Gates only actives those map areas that are "greyed out"
at that time. Say you followed this set of actions:
- Spoke to Flydian, got a greyed-out Trademeet on the map
- Stuck your head out through the Gates, turned Trademeet blue (can travel there now)
- Went to Copper Coronet, talked to Nalia, got a greyed-out de'Arnise Keep on the map
- Went from the Slums directly to Trademeet
If you did that, then the de'Arnise Keep would still be grey. You have to go to the City Gates map *AGAIN* to turn the Keep blue.