the emails I sent from gmail were either modified or the documents got
replaced for others
Where do you see the modification? In general, email does not guarantee the integrity of the messages, you need to digitally sign them to have this feature.
If you see the modifications on your Google account then someone has signed in with your credentials or used your device.
If you see the modification at the receiving end than either the receiveing end has modified the content or someone on the way (not likely).
Also, when i see the devices in recent activities, I see a WINDOWS
PHONE which is not my device, but google has only advised me to change
Changing the password is not enough if the new password leaks somehow (for instance via a keylogger or someone spying at you). You need to enable multifactor authentication. In that scenario, someone having your login and password will not be able to log in from a new device - they will be required to also provide a number which changes every minute.