Actually, you can erase and register new or another Gmail account, but you have to do a Factory Reset. But if you do this step, you can erase almost anything on your phone, like contacts, photos, messages etc.
I have a simple tutorial how to remove your GMail account on Android Market without doing Factory reset. OK, follow this step :
1. Download Root Explorer from Android Market
2. Explore to /data/system/ and find this file : accounts.db
3. Delete that file (accounts.db)
4. Reboot your Android device.
5. Walla…you can register another account