The author of 'Autocomplete' never responded. I posted the issue in a google forum. Mike there suggested I try going to the gmail account using Firefox rather than Safari.
Did that and the keychain worked like it should, entering the name and password. So we know for sure this is a Safari issue. I'll post what I've done to fix this.
Steps taken:
1. I deleted all the Keychain entries for the gmail account. This made no difference. After the information was restored to the Keychain file it still refuses to apply the password.
2. I used Keychain's "Add new password item" function to add the gmail account. It added it, but won't auto-fill the password.
3. I ran Keychain First Aid, both Verify and Repair. It said nothing is wrong.
4. I changed the gmail password, then logged back in to gmail, entering the new password. It worked, and I got into the gmail account. But the new password is also not bring filled in automatically.
I'll mention again that this is the only account and only machine on which this is happening. My other email account and a score of other auto-logins are working perfectly with Safari on this machine. And my other Macs login to the gmail account just fine with the same settings used on this machine.
Is this an unreported bug in Safari 5.1.10?