If you are referring to ‘erg’ mode, the gearing really doesn’t matter. In erg mode Zwift is looking for a certain power output from the rider, so if you change your cadence and/or shift gears the trainer resistance will adjust automatically to compensate so that your power output remains constant. (There are, of course some fluctuations, and the trainer also takes a moment to respond to each change.)
As for gearing, Gerrie is right that you should use what you are comfortable with. If you look at the ‘Difficulty’ slider in the settings, 100% will equate to feeling the gradient changes as you would outdoors (or at least as closely as Zwift can mimic them). If you slide it all the way down to zero you won’t feel any gradient changes at all (in the trainer). And in the middle (50%) you’re sort of splitting the difference. Zwift’s example is that with the slider at 50% an 8% grade in-game will feel like a 4% grade IRL. Whatever difficulty setting you choose, though, Zwift will still require the same wattage output to make your avatar move at a particular speed on a particular grade.
So why even have the difficulty slider? The reason I hear most often is that some people just don’t have the gearing on their bikes to tackle the steeper grades in Zwift at the 100% level in a comfortable manner. So, virtually decreasing the grade allows them to pedal at a reasonable cadence (even if their avatar might just be crawling along in-game).