Testei no Mint não esta funcionando o gco atualmente.
Acho que deve ser o usuário comum o problema ou a versão do MySQL.
Pré-requisitos
PHP5
MySQL5
Apache2
https://sourceforge.net/projects/godontologico/
https://sourceforge.net/projects/godontologico/files/01%20-%20GCO%20-%206.0/gco_smile_6_4_atualizacoes_completo.zip/download
# cat /etc/os-release
PRETTY_NAME="LMDE 3 (cindy)"
NAME="LMDE"
VERSION_ID="3"
VERSION="3 (cindy)"
ID=linuxmint
ID_LIKE=debian
HOME_URL="https://www.linuxmint.com/"
SUPPORT_URL="https://forums.linuxmint.com/"
BUG_REPORT_URL="http://linuxmint-troubleshooting-guide.readthedocs.io/en/latest/"
PRIVACY_POLICY_URL="https://www.linuxmint.com/"
VERSION_CODENAME=cindy
DEBIAN_CODENAME=stretch
# php -v
PHP 5.6.39-1+0~20181212060524.8+jessie~1.gbp4260ff (cli)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2016, by Zend Technologies
# mysql -V
mysql Ver 15.1 Distrib 10.1.37-MariaDB, for debian-linux-gnu (i686) using readline 5.2
# apache2 -v
Server version: Apache/2.4.25 (Debian)
Server built: 2018-11-03T18:46:19
# cat /etc/php/5.6/apache2/php.ini | grep display_errors
display_errors = On
# sudo chmod 777 -R /var/www/html/gco/
$ mysql -u root -p123
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
# mysql -u root -p123
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 37
Server version: 10.1.37-MariaDB-0+deb9u1 Debian 9.6
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]>
http://localhost/gco/
Fatal error: Call to undefined function mysql_connect() in /var/www/html/gco/lib/func.inc.php on line 46
46 $conn = mysql_connect($server, $user, $pass) or die(mysql_error());
Warning: mysql_connect(): Access denied for user 'root'@'localhost' in /var/www/html/gco/lib/func.inc.php on line 46
Access denied for user 'root'@'localhost'