One thing common between different OSes is that they provide an abstraction over graphics hardware to make developers' lives easier.

On Unix we have X Window system. On Win32 platforms - GDI. On Mac OS X - Quartz.

Since GTK+ is supposed to work on multiple platforms, it has a library called GDK (GTK+ Drawing Kit) which isolates GTK+ from the details of the windowing system.

XWindows XDrawPoint()
GDI SetPixel()
Quartz You don't wanna know
GDK gdk_draw_point()
PHP-GTK gdk::draw_point()