For doing this you have to configure your gmail account.
Mail forwarding is free in gamil and its paid one in yahoo mail.
Step 1: Goto Settings in Gmail
Step 2:Select Forwarding POP/IMAP tab
Set your Primary mail id in forwarding field, you can also have option to keep a copy in gmail inbox.
Step 3: Save settings your forwarding activated.
You can also forward some of your mail using Mail filter.