The reason that you cannot save entries in your Google calendars stored inside your CRM account is to do with the 1 way sync.
Any new entries and changes made to the Google calendar MUST be done inside Google calendar and not inside your CRM account 'Google Calendar'.
If you try entering new calendar events or make changes in your CRM account under the Google calendar it will not work correctly.
This is because your CRM account pulls calendar information in from your Google calendar into your CRM account. When this 1 way sync happens from Google i.e. hourly or 6 hourly (depending on how it is set up) it will wipe anything in your CRM account that you have entered in via your CRM account as it is only 1 way sync.
The Google calendar is a readme only format inside your CRM account so although it looks like you can type in - it will not save it.
However, we can turn off the read from the Google calendar if you wish to stop pulling your Google calendar events into your CRM account just click here to see how.