If you operate multiple calendars through Gmail, it can be a pain syncing them up to a mobile device. But there’s a way you can do it.
First, you need to open a new Exchange account through your iOS device. Put in all your Gmail details, except use “m.google.com” as the server.
Once that’s done, head to “m.google.com/sync”. There, you should be able to select your device and then check which calendars you want to sync up with your device – you can select up to 25.
Just make sure you check back on your iOS device that everything worked.