Okay, I got desperate and watched the email client closely.
The first time on the network, every update works fine.

Then, when the update interval is up, it shortly shows the "Updating...",* then immediately stops.

So somehow, the mail client wants to update, but fails. Manual update always works. Conclusion: Something's wrong with the client, it seems.

*or whatever the message is with english localization

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Update:
I've set the update interval to 5mins. Most of the updates fail, so it doesn't eat much battery. The few that work happen often enough to let me know about new email.

Are you connected to a network at all times? I believe the client fails (and stops?) to sync if no connection is present upon an automated sync attempt.

Always connected.

Interestingly, when set to 5min interval, successful connection rate is quite good. Setting it to anything longer makes the quota drop.

Whatever. 5mins it is.

For testing purposes, I set it back to 10mins. Guess what, last successful check was hours ago. I won't try to understand this anymore.

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!