On debian (based) systems, changing session.gc_maxlifetime at runtime has no real effect. Debian disables PHP's own garbage collector by setting session.gc_probability=0. Instead it has a cronjob running every 30 minutes (see /etc/cron.d/php5) that cleans up old sessions. This cronjob basically looks into your php.ini and uses the value of session.gc_maxlifetime there to decide which sessions to clean (see /usr/lib/php5/maxlifetime).
You can adjust the global value in your php.ini (usually /etc/php5/apache2/php.ini). Or you can change the session.save_path so debian's cronjob will not clean up your sessions anymore. Then you need to either do your own garbage collection with your own cronjob or enable PHP's garbage collection (php then needs sufficient privileges on the save_path).
Why does Debian not use PHP's garbarage collection?
For security reasons, they store session data in a place (/var/lib/php5) with very stringent permissions. With the sticky bit set, only root is allowed to rename or delete files there, so PHP itself cannot clean up old session data. See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=267720 .