I don’t think there would be anyone that doesn’t use Gmail. It’s like home to most of our most important and needed documents, contacts, and files. Maybe this is why it is also the home for hackers and interlopers. Perhaps this is why you might feel this sometimes there’s also someone else who's using your account. In fact, there are a few things through which you can see if there’s someone else using your Gmail account or not:

• You received authentication codes for login.
• You saw some sent mail that you don't remember drafting.
• There have been some changes in your account settings.
• You don’t get notifications of your Mails.

There can be any other activity too but here are the solutions to check the login activity of your Gmail account. 

Checking Login Activity of Your Gmail Account

Here are the steps to check the login activity of your Gmail account:

• Visit the Google activity page on your web browser and in the upper right corner, go to Google Account. 
• Sign in to your account.

Note: If you can’t sign in to your account because the password has been changed, click on “Forget password”.

• Once logged in, click on “Security”. 
• Scroll down and under the “Your devices” heading, click on “Manage devices”. 
• There, you’ll be able to see the complete lists of devices through which your Gmail account has been used. And if any of them is not yours, you can know that your account was hacked and you can make the desired changes.

Once you make sure that there’s definitely some mishappening with your account you can simply change your account password, backup email, or phone number and also see if there have been any changes made by the hackers.