Gnome and lxde are written in C/(gtk+) and xfce is written in C/(gtk+2)
It's all gtk2. gtk-2.xx.x : Names = gtk+ gtk+2 gtk2 gtk2-2.0.
The old style gtk+ = gtk-1.2.10 = gtk"1" : The last version
released year 2001, is still in use for old applications.
Gnome-1.4.xx was based on gtk+1.2.10.