The latest hg sources of gnumeric-pure works with Gnumeric 1.12 and GTK3. However, using the GTK3 version of GtkGLExt available at https://github.com/tdz/gtkglext the gl-example.gnumeric example triggers a BadWindow X11 error while executing gl_config_cb in pure-gnumeric.c:
(gnumeric:6034): Gdk-ERROR **: The program 'gnumeric' received an X Window System error. This probably reflects a bug in the program. The error was 'BadWindow (invalid Window parameter)'. (Details: serial 1095 error_code 3 request_code 1 minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the GDK_SYNCHRONIZE environment variable to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.)
This happens while the gtk_container_add() call in init_gl_window is being executed. gdb backtrace attached.