I have a solution how-to prevent automatic firmware update with Garmin Express related to this topic. I hope I don't break some forum / Garmin rules because you need to make little modifications to your firmware.
1. First of all you need RGN_Tool (use Google...)
2. Open your firmware in RGN_Tool and set "SW Version" to latest version available from Garmin, you need 330 for version 3.3 and etc. be sure to save modified file in new location.
3. If you already have original Garmin 3.3 firmware on your device, you need to downgrade to your preferred version, 3.1 in my situation.
4. Now it's time to "upgrade" to your modified firmware.
After this steps Garmin Express doesn't put new "gupdate.gcd" every time I connect my device to PC. Problem solved for me.