I just set up a GMail account & imported my yahoo contact list.?

OK, log into yahoo like you normally do and check your email. (I use classic yahoo so this is what I'll be using to tell you what to do)

At the top right just under the Web Search button is the Options link. Click that and drop down one and click the 'mail options' link. From that page click the Account Information link located on the left at the bottom of the account management links.

You'll be asked to re submit your password for security.

Now look at the area under your avatar picture. "Contact information"

You'll should see two email sources. "Yahoo! Mail" and "Email"

Yahoo! Mail is your email address that Yahoo will used to place emails sent to your Yahoo account. If I'm right, the email listed there for you will be your gmail address.

Simple click on the "Change" link to the right of this email to change it.

If this isn't the problem feel free to email/YIM me and I'll help with other remedies.