Yes, LXDE uses GTK+ just like GNOME and XFCE.
AFAIK, what makes LXDE different (and faster!) is that they are mostly written in pure GTK+, rather than using the GNOME or XFCE libraries. Also, pure GTK+ apps tend to have better performance, as they are compiled C code. Where as other GTK+ bindings such as PyGTK, GTK#, etc. tend to be "easier" to code and develop, but lose somewhat on the lightweight side.