I have been on the security manager page and although the device is there, there is no option to remove it or deauthorise it. I have changed my password but am unsure if they can still gain access without needing a password as I have just clicked on my phone to gmail and it opened without needing a password. I'm not good with computers so don't know what else to do. Help would be appreciated x
If you changed your password, your account is signed out everywhere and requires the password to log in again. All is safe.
Ethan's answer is correct, but for peace of mind: You can also log in to Gmail on your PC, hit "Details" at the very bottom-right of the page, and choose "Sign out all other sessions".
If you are really worried about security you can go for Google two step verification
Everytime you sign in from any device or anywhere you will be asked for OTP. It gives another layer of security to your account.