You want to decrypt a file that was encrypted with GnuPG.
Assuming the file is myfile.gpg, decrypt it in place with:
$ gpg myfile.gpg creates myfile
Decrypt to standard output:
$ gpg --decrypt myfile.gpg
Decrypt to a named plaintext file:
$ gpg --decrypt --output new_file_name myfile.gpg
These commands work for both symmetric and public-key encrypted files. You'll be prompted for a password (symmetric) or passphrase (public-key), which you must enter correctly to decrypt the file.
ASCII encrypted files (with the suffix .asc) are decrypted in the same way as binary encrypted files (with the suffix .gpg).