Refer to the link you pointed out in your post :-
'The IP you're using to send email is not authorized...' - Gmail Help
Seems gmail is doing a reverse lookup for your IP. Would you mind having a PTR record ?
> set type=PTR
> 61.246.236.78
Server: MBMUMXXXXD001DNS005
Address: 124.124.5.140
Non-authoritative answer:
78.236.246.61.in-addr.arpa name = ABTS-TN-Static-078.236.246.61.airtelbroad
band.in
236.246.61.in-addr.arpa nameserver = dnsblr.mantraonline.com
236.246.61.in-addr.arpa nameserver = dnsglobal.mantraonline.com
236.246.61.in-addr.arpa nameserver = dnsbom.mantraonline.com
236.246.61.in-addr.arpa nameserver = aaadel.mantraonline.com
236.246.61.in-addr.arpa nameserver = dnsdel.mantraonline.com
aaadel.mantraonline.com internet address = 202.56.230.6
dnsbom.mantraonline.com internet address = 202.56.240.5
dnsdel.mantraonline.com internet address = 202.56.230.5
dnsglobal.mantraonline.com internet address = 202.56.230.52