I am aware that this community post is old but I have been playing around with the settings for Gmail Sync of Email, Calendar and Contacts.
I can confirm that I have success in all Sync areas!
I have Gmail Email syncing
I have Gmail Calendar syncing
I have Gmail Contacts syncing
I can explain how below (please read on)
Removed all calendar entries from N900
Removed all current email accounts from N900
Removed all current contacts from N900
Note: Backup all current data prior to removal (export contacts, etc)
Configure Gmail settings in Mail for Exchange as follows;
Server "m.google.com"
Port "443"
Conflict resolution "Server takes priority"
Username "xxxxxxxx" no @gmail.com
Password "xxxxxxx"
Domain Leave blank
Synchronise email "Tick/Enable"
Account Title "As desired"
Synchronise messages back "1 week" (not to load intensive seems to work better)
When sending e-mail "Send immediately"

To successfully get gmail syncing you also require Mfe folders installed.
I have found that to enable successful Gmail Email Syncing you require to remove as much as possible so Mfe folders will enable you to remove folders not require during sync, I have removed Spam, other email accounts inbox which are already synced with the all mailbox folder.
I now have successful sync of Gmail Email along with the other Syncs working great.
As stated I know this posting thread is 2 years old but I have yet to find anyone who has success with this and who has posted the success for all to share.
Happy Syncing!