Just tried to send via Gmail both on Linux and Windows 10, it worked fine. This is the configuration I've used:
Server=smtp.gmail.com
Port=587
TLSMode=STARTTLS
[email protected]
Password="xxxxxxxxxxxxxxxx"
[email protected]
FromName=NetXMS Server
IsHTML=no
Interesting part is Gmail shows app password with spaces, but password should go without spaces.
Is time on your Windows machine correct? What kind of Windows it is? You can try installing NetXMS server on another Win machine and copy notification channel configuration there.