Gmail Sending Failed

Just to be sure, please check if your IP is not blacklisted:
https://mail-blacklist-checker.online-domain-tools.com/
https://mxtoolbox.com/SuperTool.aspx
https://www.blacklistmonitoring.com/lookup/run_blacklist_lookup_tool.php

Checked twice. I address and domain are not blocked. Checked on mxtools.

Okay, then you can check the spam score of your newsletters, maybe Gmail doesn’t like them. Online tools:
https://www.mail-tester.com/
https://isnotspam.com/
I hope it helps.

Is there any specific need of sever for the plugin to work ?

No specified needs more than WP requirements.