Ragespline work great with PlayMaker, but I'm not sure what you mean by "combine with PlayMaker GUI"?
Unity has a few different approaches to UI.
Actions in the GUI and GUI Layout categories use OnGUI, a separate 2D GUI system that draws on top of the 3d view.
GUI Element actions expose the older GUI Text and GUI Texture systems, fine for simple texture based GUIs.
Then you can turn any game object into a simple UI element using collision boxes and mouse events (e.g. MOUSE DOWN). I think this is the approach you'd need to take with Ragespline.
Someone with more experience with Ragespline might have more tips...