( Originally noted running Go 1.8.3 locally on MacOS Sierra 10.12.5.
Looking at current on-line documentation at golang.org/pkg/compress/gzip. )

The documentation of Close() method of gzip.Writer says that it flushes any unwritten data and implies that it does nothing more. That's not quite correct. Critically, it also writes a termination sequence to the output file. Without this, the corresponding Reader will return an "unexpected EOF" error.

I think that's a significant omission. From the documentation I concluded that Flush and Close were equivalent, and I initially chose the wrong one.

Sample test program:
https://play.golang.org/p/zPOjLCAttg

The same situation also exists in the very similar compress/zlib package.