I have a Google calendar witch syncs automatically with my iMac.

Items from this calendar don't get put on the iPad with I sync it and I can't see any way to have Calendar on the iPad sync with Google calendar.

How can I sync Calendar on the iPad with a Google calendar?

I currently use Calendars by Readdle, very easy to use with all ical calendars... including Google.

You need to set up your google account as an Microsoft Exchange Server account in the mail settings section. Her is a link to a google support article, or you could just google it.
Set Up Your Apple Device for Google Sync : Mail, Calendar, & Contacts - Google Mobile Help