With a commit made on Friday, Eclipse's SWT platform is now using GTK 3.x by default over GTK 2.x.
The SWT GTK3 support by default is used when the GTK3 support is available and only for Linux/Unix platforms. This change to use GTK3 over GTK2 will be found with the Eclipse 2014 "Luna" release.
The change happened with this Git commit
for the Eclipse platform and more details can be found by this bug report