This is not at all a complete troubleshoot guide and depending on what error you are experiencing this might not be the easiest way. But, when you googled for your support code or problem and didn't really find anything usefull this might be a great place to start out. I won't explain everything in the tiniest detail, if something is unclear please let me know and I'll dig further down.
First of all, back-up. With that I mean both on your device and your data tied to your Google Account. Normally you don't risk your google information at all but better be safe then sorry.
First and probably easiest step, start-all over!
Do this by either going through the Google step-by-step or my own WM 6.1 version (note that my guide was made before GMail push was supported, just ignore that). My instructions below, click it for a better view.
So any luck, no? Then let's move on.
Next step is merely an extra validation, do you have Tasks selected?
Tasks is not supported by Google Sync for Windows Mobile and will mess things up if enabled. So double-check! Start-> Program-> Connections-> Activesync and check your settings.
Next step is to see what actually do work and where you have your problem.
To do this you go back to the settings where you just checked that Tasks wasn't enabled. First we try to synchronize nothing but air, we do this by deselecting every single type of data. Deselect Contacts, Calendar, Mail and the rest should always be deselected. Now try to synchronize. This really should work otherwise your set-up is most likely not correct. If it works, good. We now know that you have a working connection to the Google Servers.
This is the screen you should be at, just deselect everything:
Now we continue to rule out causes for your problem.
Enable Contacts (only contacts). Does it work? Great, now we know that Contacts isn't your problem. Continue by deselecting contacts again and select calendar instead. You get it, try them one by one.
This is the time where the first weirdness often shows up. Did they all work separately? Strange...try them together! Many users reports that after doing the separate sync they can use them all together, weird weird but if it works don't question it :)
If everything didn't work you hopefully only got one type of data that didn't (otherwise take the following steps for each affected type).
Calendar didn't work? Follow me into the hardest and definitely less fun step.
Now we want to start with a clean calendar and gradually populate it with your events again. If you have everything in your Google Calendar it's not that bad and you can just skip this step concerning device calendar.
The device calendar can contain the bad seed just as well as your Google calendar. Therefor you should back it up (sync with outlook or the calendar software of your choice) and then make sure it's completely clean.
To clean your Google calendar without having to start all over just follow these steps. We are now going to take advantage of the fact that Google Sync doesn't support multiple calendar sync. Create a new calendar from your computer, now in one way or another copy all your events from your standard calendar to that one. There are multiple ways to achieve this, synchronize it with Outlook or export your whole calendar, do as you please. If you want more details on how to export your calendar just give a comment and I'll update with some additional info. Tada! Your synchronizing calender is clean.
Try a sync, if it works which is often the case you got yourself an "per-item error" or as you might put it, some of your events are to blame for your tears. There are no easy way to know which one. The way I've solved it was to add my old events back in batches of 50 events (if you have lots and lots of events you might wanna have bigger batches), when the sync stopped working I knew that in the last batch something is fishy. Smaller batch, and then I found 10 events with messed up categories.
If your calendar was to blame it should hopefully work now.
This step is for you who had the problem in Contacts, it's more or less the same as for the calendar.
First make sure that your device is free from local contacts (sync with Outlook for example). Then login to your GMail account and go to Contacts. Create a new group by selecting the +Group-symbol (marked with "1" in the picture), let's call it "Temp".
Now you are going to assign that group to everyone in your "My Contacts" group. To do this open "My Contacts" then "Select all" then just select "Temp" below "Add to...".
"My Contacts" and "Temp" should now contain exactly the same people. Go to "Temp" and remove the "My Contacts" group from everyone (same procedure as above but "Remove from... My Contacts". As "My Contacts" is the only group being synchronized you will now sync 0 contacts. Try it, it should work unless your connection set-up is messed up. Now start adding them back to "My Contacts", depending on how many contacts you have you can start out with anything from 10-100 but more than that it will get a bit out of hand. When the sync stops, you know that something in that batch messed it up. Split it into smaller batches untill you find your problem. Either you can see that one/several field looks strange, or you can choose just to re-create those contacts that won't sync.
Most of those "per-item-errors" originates from events or contacts created/synced with other software or even hardware. In my case it was old Palm events that was synced. Sometimes however you can't really tell why a post won't sync, this is why they call it beta I guess :-)
If your problem was with the mail sync I can't say much. I switched from Windows Mobile before that was supported but you can always try my freestyling skills and ask anyway :) Feel free to comment if any part was extra useful or a waste of time. If you can't get it to work I will gladly see what I can do, so post a comment or send me an email and I will answer.