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Google Calendar Sync seemed to be missing come random calendar entries while syncing with Outlook. I'm using Outlook 2007 SP2 and Google Calendar Sync 0.9.3.6. On much Googling around I found following two solutions (combined together) work for me.

Before we jump into the solutions check the sync logs, usually located at below path. This will give an indication of what might be going wrong.

C:\Documents and Settings\<USER_NAME>\Local Settings\Application Data\Google\Google Calendar Sync\logs

Calendar Message Class Should Be IPM.Appoinment

For Google Calendar Sync to work correctly, your calendar appointments message class should be IPM.Appoinment. If it is not this EXACT message class it will not sync. A different message will occur if someone has used a custom form to send you an appointment request.

To view your message class:
Open Outlook Calendar on the computer.
For XP: Choose View > Current View > Events.
For Vista, choose View > Current View > Customize Current View.
Choose View > Arrange By > Current View > Customize Current View.
Click the Fields button and choose All Appointment fields from the pop-up menu in the "Select available fields from" section.
In the "Available Fields" column, select "Message Class" and click the Add button.
Click OK, OK. This will show you what each appointments message class is.
See any correlation to missing appointments and them being something different than IPM.Appointments? They probably look like: IPM.Appoinments.Something.

Next step: Change your appointments back to the standard message class.

Apparently, there are numerous of ways to do this, but the easiest way is by using program: DocMessageClass. Downloaded it from the site: http://www.publicshareware.us/public-share-outlook-utilities.php

Just run the application (no installation required) and select your calendar folder. It will populate the list of all available message classes. Select all EXCEPT for IPM.Appointment and convert them into IPM.Appointment. BANG! Successful sync of all my events.

Missing Address Entries

If the Outlook Offline Address Book is from a different source than the Global Address Book then some of the calendar entries fail to sync.

In such (or similar) case, you’ll see "SEVERE: Recipient was not found" message in the log entries. This is due to an email address that was selected from the Global Address Book, not being found in the Offline Address Book. This seems to create an email resolve error, when Google Calendar Sync begins to process the calendar items.

To fix the issue in Outlook 2007, follow the below steps:
Go to: Tools > Send/Receive > Send/Receive Settings > Define Send/Receive Groups.
Click "Edit" on "All Accounts"
Click "Address Book Settings"
Under "Choose address book" select "Global Address List" if something else is selected. Click OK, OK and then Close.
Go to Tools > Send/Receive > Download Address Book
Unchecked "Download changes since last Send/Receive" and make sure "Choose address book" was set to "Global Address Book". Click OK

Troubleshooting issue becomes much easier is you turn on verbose logging. Create a file called "level.txt" containing the single word "VERBOSE" (without quotes) into the Google Calendar Sync\logs directory. This will turns on verbose logging.

Update: Still struggling to resolve error:

SEVERE: The user has exceeded their quota, and cannot currently perform this operation

Update: I got rid of exceeded quota error by reducing the frequency of update to 6 hours. Understand, this is just a workaround and not a real solution but it sure helps!