If your host allows you to, you can set these in a .htaccess file when you don't have access to edit the php.ini (although I'm unsure as to what level of permissions you'd need to override this setting)
.htaccess code:
session.gc_probability 1 1
session.gc_divisor 1000 1000
To check if this actually works wiht the security level they allow you (I use AllowOverride ALL in my php.ini for my sites so i can override settings as needed) you'll need to open a phpinfo script to check.
phpinfo.php:
