I don't have any experience with your particular situation, but I would guess that regardless of window decorations, it's up to your window manager to allow movement of the window. (Like you said, it's alt-click for some, or you can grab the titlebar, or similar things).
For apps that have no decorations, you probably need to handle the click event yourself (since your window manager doesn't concern itself with clicks inside the application area). For example, designate some area on your app where you want to be able to "grab" to move it, and look for a mouse-down event there.
Though, once you get a mouse-down, I would have no clue about what code you can use to move the application... I'm not sure if applications are allowed to do that, or if the window manager is supposed to be responsible for it. So I'm afraid I can't be of too much help

Maybe this'll give you some ideas though.
Of course, as macewan suggests, check out the source code for other GTK apps that do what you want. That's the beauty of open source!