First thing, the settings for sending must be correct.
The user name/password are what is used to log in to the NAS shares from a computer on the network.
For the port and SMTP Server see
https://support.google.com/a/answer/176600?hl=en Although they mention printer etc. it is the same for the NAS.
The sender should be something to define the NAS (example: my.nas@myhome.com)
SMTP Auth - see the info in the above link.
Finally do a test send to check it is up and running.
Don't forget to
save settings and also set what information you want to get from the NAS box.
The receiver is your gmail name@gmail.com