That’s not how virtual shifting works in Zwift. It could be, if they wanted to do it, but the way they implemented it depends on a trainer firmware update so it knows how to put you in the middle of the gear range. The QZ app (qzfitness.com) does allow you to manage resistance in the way you described, and it can be paired with the Zwift Play controllers for shifting (though I think the other functions of the shifter are not supported). Talk to the developer @Roberto_Viola if that interests you.