Gmail has a free email forwarding that send all inbox to your main Gmail account.
To setup this settings Log in to your secondary Gmail account.
Go to Setting on the top right corner of your Gmail page
In Settings click Forwarding and POP/IMAP tab
Then in Forwarding: put your main account wherein you want to receive all the emails from your multiple account then select keep Gmail copy from the inbox
That’s it all your emails from your multiple Gmail account will now be sent to your main Gmail account, and yes, not like Yahoo! Gmail offers you this for free!