Make sure you have your Google account added to your Calendar in the Settings (that's the Gear icon) of your iPP. Don't even open the Calendar app at the moment.
Open 'Settings', tap 'Calendar', tap 'Accounts', then add your Gmail account. Once it's added, tap it and make sure Calendar is selected. I'd also suggest you tap your iCloud account and turn the iCloud calendar off (it may spit out a warning about deleting the iCloud entries, but that's what you want), so that only your Google account controls the calendar.
Once that's done, check your calendar and see if its synced properly now. It should be.
I only use my Google account for my calendar and I have both Android (Nexus 6P, Pixel C) and iOS (iPad Air, iPod Touch 6) devices and everything syncs properly between all four devices if I add/change/delete an event on any one of them, so it doesn't matter if you have an Android phone and an iPP as long as the iOS settings (which are a tad un-intuitive honestly) are configured properly.