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).
gpg(1).