I ran the file command against a file and got this result:
data (gzip compressed data, was <fd:1> from unix)
What does the "<fd:1> from unix" mean?
fd:1 is "file descriptor 1", which means standard output.
My guess is that the file is the result of a pipe from the output of another file like
cat file|gzip - > new.gz
I was not able to reproduce that output, but it could be that different versions of gzip handle that differently.