two fixes I've found on the net and am still trying:
1. clear the cache in Settings>Applications>ManageApplications>All>Gmail
2. if low on space, gmail app stops syncing correctly. So, uninstall/move stuff onto SD card from phone. Also, I'm not 100% on this, but I think that you can clear the data from "applications" like "com.htc.socialnetwork.provider" and things will probably still be ok...this is what I had to do to create enough room.
Am annoyed by the lack of internal space on my Desire (original), and also at the number of apps that are obliged to install to the phone and cannot sit on the SD card (eg. google maps, gmail, facebook, Internet etc)