In the panel, here: https://panel.dreamhost.com/index.cgi?tree=support.msg&
Frankly tho, there is a better way to do what you want to do.
Sadly you will always have trouble with Dreamhost forwarding mail to gmail (as well as others).
What I would do if I were you:
1 - Convert the email address back to an email box in the dreamhost panel. Don't set up any forwarding.
2 - log into your gmail account.
- Use the gear in the top right to select "Settings"
- Click on "Accounts and Import" near the top, just left of Center.
- Click on "Add a POP3 mail account you own." and follow the prompts. You will need to give the password to the dreamhost email account.
This will let gmail collect your email out of the dreamhost account, rather than asking dreamhost to forward it to gmail. It will work this way, whereas forwarding will always be problematic.