GBT 7.2/7.3 are a complete rewrite of previous GBT versions (<7) with regard to polytope and ellipsoid operations and there is no code shared with GBT 6.1 or previous versions. To achieve high numerical reliability the whole package has been changed.
The authors of the new algorithms and MATLAB code are:

Professor Sandor M Veres, University of Southampton, Southampton, UK

and

Professor David Q Mayne, Imperial College of Science and Technology, London, UK