Why is it impossible to move a window programmatically in GTK+ (C/C++)

[mrt303xx aol com]

I am writing an app in GTK+, and I wanted to center a splash screen.  Coming from a Windows background, where 
SetWindowPos does the job, I found out the X Window Manager always ignores requests to move a window, 
therefore eventually denying any attempt to manipulative the position of a window.  I can resize it, even 
dynamically, and I can move it with the mouse, but never in code.  Why?  It seems very strange that GTK would 
just disallow it.  You have gravity properties, but not matter what gravity I set, the position NEVER 
changes.  Why have the APIs if GTK is free to ignore them.  I'm baffled by it.