Installing on Debian
Debian 6 doesn't have GTK+ version 3, and Debian 7 has partial support for all the features we use in GTK+ (version 3.4 while our minimum is supposed to be 3.4). You can still make it work with Debian 7 but Debian 8, codename Jessie is recommended. For a desktop system, the next stable is probably the better choice, called Debian testing, currently version 9 with codename Stretch.