How to apply Zip Compression in GnuPG Class of PHP

I would like to compress message using GnuPG PHP extension

 putenv("GNUPGHOME=/var/www/.gnupg");
 //$gpg = new gnupg();
 $res = gnupg_init();
 $rtv = gnupg_import($res, $pubkey);
 gnupg_addencryptkey($res, $rtv['fingerprint']);
 $pgp_str = gnupg_encrypt($res, $str);

I would like to pass compress-algo parameter but haven't found way to pass, can someone suggest?

Thanks for help!