Unfortunately, this isn’t possible with Smart BCC in the Silver plan. The best solution in this case is to upgrade to Pipedrive's Gold plan and use full email sync.
Auto-forwarding is not possible as Gmail needs to send a confirmation email to enable auto-forwarding. The confirmation that Gmail sends is blocked by our system as it is from an automatically generated unique email address each time. As the confirmation email is from an unknown address in Pipedrive, you cannot confirm Google's security requirement.
Aside from upgrading to Gold, the only other solution is to manually forward each email reply to your BCC address.