If your other email account is also a Gmail account, go into Gmail’s settings, click over to the Forwarding and POP/IMAP tab and click Add a Forwarding Address.
You’ll be prompted to enter your main email address. Once you have, you’ll receive a confirmation link at your main email address. Click it to confirm that you own your main account.
After confirming, you can use the Forwarding options on your other account to forward all email to your main email address. Make sure you click the Save Changes button after.