iPhone Calendar not able to 'Accept', 'Maybe' or 'Decline' from a Google Calendar invite

Hello Simonray,

It sounds like you need some help sending calendar invitations. You can learn how to do that here: Send invitations in Calendar on iPhone - Apple Support

We think you'll find these steps helpful:

You can invite people to an event you’ve scheduled.

1. Tap the event, then tap Edit near the top of the screen.

2. Tap Invitees.

3. Enter the names or email addresses of people you want to invite, or tap  to select Contacts.

4. Note: For you to send a calendar invitation to a contact, they must have an email address. See Add and use contact information on iPhone.

5. When you’re finished, tap Done.

If you don’t want to be notified when someone declines a meeting, go to Settings  > Calendar, then turn off Show Invitee Declines.

Note: With Microsoft Exchange and some other exchange servers, you can invite people to an event even if you’re not the one who scheduled it.

Thanks.