according to the error message [file location and name].gpg does not contain encrypted content but OpenPGP keys. those can be imported, but not “decrypted”.
you can open a command line and call gpg [file location and name].gpg without further arguments to examine it.
It was a collection of documents zipped and then encrypted by one of our contacts for two different addressees. One had no problem and their GPG installation decrypted it without issues. The one in this computer didn’t and complained as reported.
FWIW, regarding the suggested command, i did not include -d or any other option on purpose. gpg tries to guess what to do with a given file if no particular option was given. in this case, however, you would have seen the same error message, and i think that gpg should do better than returning “unknown system error” if it was given an empty file.