I was recently introduced to DistroWatch.com and found it very useful for being able to figure out what versions of software are in use. The following table of GNOME releases is based on data from this site. This table helps guess at the minimum GTK+ version users are likely to have, and which problems they may experience.
I last updated this list on October 13th 2005.
| GNOME 2.12 GTK+ 2.8.3 September 2005 | SuSE 10.0 | October 2005 |
|---|---|---|
| Ubuntu 5.10 | October 2005 | |
| GNOME 2.10 GTK+ 2.6.4 March 2005 | Mandriva 2006 | October 2005 |
| Gentoo 2005.1 | August 2005 | |
| Fedora Core 4 | June 2005 | |
| Ubuntu 5.04 | April 2005 | |
| SuSE 9.3 | April 2005 | |
| GNOME 2.8 GTK+ 2.4.9 September 2004 | Debian 3.1 | June 2005 |
| Mandriva 2005 | April 2005 | |
| Gentoo 2005.0 | March 2005 | |
| RHEL 4.0 | February 2005 | |
| Fedora Core 3 | November 2004 | |
| Gentoo 2004.3 | November 2004 | |
| Ubuntu 4.10 | October 2004 | |
| GNOME 2.6 GTK+ 2.4.0 April 2004 | Novell Linux Desktop 9 | November 2004 |
| SuSE 9.2 | October 2004 | |
| Mandrake 10.1 | September 2004 | |
| Yellow Dog Linux 4.0 | September 2004 | |
| Gentoo 2004.2 | July 2004 | |
| Fedora Core 2 | May 2004 | |
| GNOME 2.4 GTK+ 2.2.4 September 2003 | Mdkmove2 | May 2004 |
| Gentoo 2004.1 | April 2004 | |
| SuSE 9.1 | April 2004 | |
| Mandrake 10.0 | March 2004 | |
| Gentoo 2004.0 | March 2004 | |
| Mdkmove | December 2003 | |
| Fedora Core 1 | November 2003 | |
| Mandrake 9.2 | October 2003 | |
| GNOME 2.2 GTK+ 2.2.1 February 2003 | Sun JDS 2 | May 2004 |
| Sun JDS 2003 | December 2003 | |
| RHEL 3.0 | October 2003 | |
| SuSE 9.0 | October 2003 | |
| Gentoo 1.4 | August 2003 | |
| RHL 9.0 | March 2003 | |
| Mandrake 9.1 | March 2003 | |
| Yellow Dog Linux 3.0 | March 2003 | |
| GNOME 2.0 GTK+ 2.0.2 June 2002 | SuSE 8.1 | September 2002 |
| RHL 8.0 | September 2002 | |
| Mandrake 9.0 | September 2002 |