In Settings → Accounts, I have another account for Send mail as. And in Forwarding and POP/IMAP, I enabled the auto forward.
The problem is, when Gmail auto forwarded it, didn't used the "other account". Why?
send mail as setting is used to send new mail, respond to email, or forward a message to one or more people. It looks like you are sending email from that other address.
Forwarding and POP/IMAP sends all your incoming email to another address. You use this if you want to send all your email to a new address.