It is apparently an internal site owned by Google to try and reduce spamming. See below.

What is Gappssmtp?

gappssmtp.com is one of google’s DKIM (DomainKeys Identified Mail) or email authentication gateway designed to detect email spoofing. … DKIM lets a domain associate its name with an email message by affixing a digital signature to it.

Thanks @elbanditoroso
I haven’t been on Fluther since posting the OP or so.
As an aside, I get an error message when using Firefox ’‘can’t find this site’’ and Chrome ’‘inaccessible’’ if I go by open in a new window or in a new tab. But ok when I click on it.