Try this:
In the Mail ap, click on "Show" beneath the Compose Mail icon (or in the menu bar, click View>Show Mailbox List)
In the Mailbox List that appears on the left side of the screen, "Inbox" should be at the top, with your email accounts listed beneath
Make sure that your gmail account appears at the top of the list. If it doesn't, just click and drag it to the top.
I have experimented with this method several times, and it works consistently for me.