Game OpenGL GUI

Short: How do you make a GAME GUI? (using openGL)

I'm making a multi player network game. My part of the program
concerns a basic GUI. I have no idea where to start though.. Right now
we're still using QT, but that's just for easy testing. It'll have to
go and it'll have to be replaced by a pure OPENGL user interface.

This doesn't have to be a big thing, just keyboard interaction is
sufficient. (for example browse through: new game - load game -
options - exit). I was thinking about captioring keyboard input and
accordingly show another bitmap.  This seems far fetched though!

Is this the way it is normally done? (In OpenGL) Or is it suggested to
use some libraries like GLUI, MUI, or PUI? But then degree of freedom
is very limited I bevieve.. + In the end the game should still be
platform independent.

I've searched much of the internet but I haven't found the information
I'm looking for.. Any help or pointer to the right direction would be
greatly appreciated! Btw: pls excuse my spelling.