gpg <path/to/file> does return the vague error message "unknown system error" if the given file is empty. this is neither a system error, nor should the problem be considered "unknown".

gpg should rather return something helpful the likes of "no content".

this was mentioned in the forum for windows and i was able to replicate it on linux with version 2.5.12:

$ touch test.gpg
$ gpg test.gpg
gpg: WARNING: no command supplied.  Trying to guess what you mean ...
gpg: processing message failed: Unknown system error