GDK is basically a wrapper around the standard Xlib function calls. If you are at all familiar with Xlib, a lot of the functions in GDK will require little or no getting used to. All functions are written to provide an way to access Xlib functions in an easier and slightly more intuitive manner. In addition, since GDK uses GLib (see below), it will be more portable and safer to use on multiple platforms.
One of the nice things about GDK is that it's based on top of Xlib; this is also a problem, especially in the area of color management. If you want to use color in your program (drawing a rectangle or such, your code should look something like this:
{
GdkColor *color;
int width, height;
GtkWidget *widget;
GdkGC *gc;
...
/* first, create a GC to draw on */
gc = gdk_gc_new(widget->window);
/* find proper dimensions for rectangle */
gdk_window_get_size(widget->window, &width, &height);
/* the color we want to use */
color = (GdkColor *)malloc(sizeof(GdkColor));
/* red, green, and blue are passed values, indicating the RGB triple
* of the color we want to draw. Note that the values of the RGB components
* within the GdkColor are taken from 0 to 65535, not 0 to 255.
*/
color->red = red * (65535/255);
color->green = green * (65535/255);
color->blue = blue * (65535/255);
/* the pixel value indicates the index in the colormap of the color.
* it is simply a combination of the RGB values we set earlier
*/
color->pixel = (gulong)(red*65536 + green*256 + blue);
/* However, the pixel valule is only truly valid on 24-bit (TrueColor)
* displays. Therefore, this call is required so that GDK and X can
* give us the closest color available in the colormap
*/
gdk_color_alloc(gtk_widget_get_colormap(widget), color);
/* set the foreground to our color */
gdk_gc_set_foreground(gc, color);
/* draw the rectangle */
gdk_draw_rectangle(widget->window, gc, 1, 0, 0, width, height);
...
}