Gmail opens in new window when trying to login

Gmail has been doing this for a few weeks now. It's really annoying.

There are a couple of reasons that this could be happening:

You may have more than one browser open, each with a different window open to Gmail. So when you click to log in, it opens in a new window because that's what you have specified in your browser settings.
Alternatively, you may have clicked on a link that opens Gmail in a new window.
If you're using Google Chrome, you can try opening an Incognito window (Ctrl+Shift+N) and see if that solves the problem.