There sure must be several ways to forward/copy Gmail messages from one account to another. Here we will see one of them using IMAP functionality of Gmail and Thunderbird, a free email client.
First, you will need to enable IMAP on each of your Gmail accounts: https://mail.google.com/mail/?hl=en&tab=wm#settings/fwdandpop. If the link does not work, please see Gmail [...]