[Skarg]

Cities which rely on gates?

Hopefully those are very rare and exceptional, unless you want a world where gate travel makes your world geography more or less irrelevant.

It depends on the nature and quantity of the reliance, and the availability of gate wizards. How often do they need to pass someone through the gate? And how many gate wizards do they have available/willing to help them with gates? And for how long?

In general, if you want to keep a gate available, you can have someone available who can stabilize a gate when it breaks down, or you can have more than one gate from A to B, and stop using the gates when you get down to one gate, unless you have a gate stabilizing wizard available. In an emergency, you can risk the life of a very powerful wizard who has both Create Gate and Long Distance Teleportation.

If you only mostly need the gate to exist, but not to constantly use it, then it's not that hard, because you can just call for a gate wizard when you use it, or when your redundant gates break down to the last one.

If you need steady traffic, then you start needing a dedicated corps of gate wizards.

Having a group of people who know the Aid spell is generally wanted/needed too, especially for heavy operations.