Not sure if I have the right end of the pole but do you mean all the aliases in the DreamHost Panel that go to your email user?
You could just add the gmail account in as a forwarder in the mailbox section using the edit button.
email@example.com ....also forwards to firstname.lastname@example.org
Then all the aliases in the forwarding-only section would go via your username to the gmail account.
You could group your forwarding-only aliases as well if you wanted, to specific gmail accounts.
Alternatively the Custom MX / Gmail option from the DreamHost control panel may suit your needs.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK! :@
Act on my advice at your own risk!