There's some Gutenberg affiliates set up in other countries that host content which is not public domain in the US but is public domain elsewhere (Australia and Canada have some notable additions as a result of years where they didn't line up with US law). It looks like Gatsby is on Gutenberg Australia.

You aren't imagining it. The Great Gatsby was in public domain for a few years until the Copyright Term Extension Act 1998 added 25 more years.

This created a complications because lots of classical works that were in the public domain had to be removed from sites like Project Gutenberg. Apparently, popular, long dead authors are supposed to support their great grandkids from those royalty checks.

I wrote a school book report on The Great Gatsby from the text hosted on Project Gutenberg. And when I had an interest in looking at the text again imagine my surprise to find it no longer listed.