That is not even a question that should be asked. This gate is going to be around your kid for a long time. CLeaning the gate on time will ensure that the gate remains sanitary and does not become a medium for bacterias or viruses.
In short, terms, Clean the gate every day (if not possible, then every other day).
Avoid using chemical-based cleaners. They can have harmful dermatological and chemical effects. Go for water-based cleaners. Most of the time, you would be dealing with dirt and dust, so any water-based cleaner would be enough to take care of that.