FWIW, I totally agree with Falko. I went the multi-site route with ISPConfig 2. There are so many features that don't work, that I had to go back, tear it apart and install gallery2 wherever I needed it to be. Also, be sure to name the sub-directory that you install it into as 'gallery2' so that the URL re-writes work. I was in re-write hell until I did this and then the gallery2 wizard worked fine.
So us /var/www/example.com/web/gallery2 - or if you want it by each user, do /var/www/example.com/user/web1_user1/web/gallery2