Log into your phones gmail account
At the top right of the page click settings> accounts and import tab
Two colums down it says:
Send mail as:
It shows the list of email addresses gmail can send your mail as. If your yahoo email isn't on here click "Send mail from another address" and set it up.
And under that it gives you two options, select the first one or change your default email address to your yahoo email address to always use yahoo to reply to any yahoo or gmail message you recieve.>
When receiving a message:
Reply from the same address the message was sent to
Always reply from default address (currently ______@gmail.com)
Hit Save.