Hello Pauwhoovie16
The Z-Pass OSTWIND-ZVV Day Pass for 5 Zones (120, 122, 164, 917) will cover the zones which you are missing on the route Zürich HB - St. Gallen as long as your train stops in Wil SG. That is because, according to the Z-Pass Connection Rule, network tickets are valid only up to the last timetabled stop within the zones purchased. Please check the information here.
Usually, IC5 does not stop in Wil SG. I suggest you take the IC1, so that your combination is valid. Alternatively, if you really need to take the IC5, you can purchase a Point-to-point ticket from Zürich Flughafen (which last stop in the area included in your ZVV pass) to St.Gallen and, from there, you can use the OSTWIND 9 o’clock Day Pass All Zones.
I hope that my answer is clear and wish you a love day,
Chiara_S