GUI development

[Cody Godines_1]

OK, I need lots of help and advice on this one.
I am beginning to work on a GUI for a computational program written in both C++ and Fortran 77. We have already put out several releases of the application, and now it is my turn to contribute to the GUI development. The routines are compiled and run on an HP 9000 w/ 10.20 on it. Up until now, a program called Rapid Application was used to develop a ViewKit GUI. The ViewKit has its own libraries and, if I remember correctly, makes X and OSF/Motif calls. The problem is that (1) they also need to be compiled and run on SGI and LINUX machines, as well as PC's using Reflection X. The other problem (2) is that when we use applications to make the initial GUI code, we loose some control and cannot customize certain GUI features. So, my main question is: what tools can I use (libraries, debugging, etc) to develope a GUI that is robust, controllable down to the placement of the windows, and can be compiled on other UNIX machines. Please, I need to know the best tool or set of tools that I will not run into problems later, and I don't mind the learning a new tool startup time as long as it is the best (long term code development).
Thanks and will someone ask something easy so I can answer it?
-Cody