We're so used to reliable communication channels that when something breaks down, it takes us by surprise. I've recently experienced a glitch that's been causing some headaches. Calendar invites created in OS X iCal would not get delivered to Gmail recipients. No matter what we tried, the recipient never got it.
After some research, it turns out that if the recipient is using a Gmail email address as their login for iCloud, somehow iCal invites never get emailed but end-up directly in the corresponding iCloud calendar. Most Gmail user will not check the iCal calendar and assume that the invite has been lost.
Here are some simple instructions how to fix this.
Go to icloud.com and log into the calendar, select preferences
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Select "Email to ..." option and save
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This won't affect previously sent invitations but new invitations should now be delivered into your email and picked up by Google calendar.