For instructions to setting email forwarding with Gmail refer to this article: Automatically forward Gmail messages to another account.
Important: Gmail verifies that the user has access to each email address being added as forwarding address. As part of this step, anytime a new forwarding address is added to Gmail, it sends a code to the new forwarding address. User is expected to extract code from the email (sent by google) and enter that within verification code box (within Gmail settings). To address this, following tips can be used: