Proprietary code that wasn't developed and tested with generic requirements in mind can rarely be economically re-used outside of the original company context, as it reflects the structure and the processes of the mother-company. In fact most organisations I know decided to throw away their code base entirely when restructuring or upgrading their internal systems as re-work was deemed uneconomic, because the gap between the code and changed environment is too great.
Data is salvageable and immediately valuable; however re-using the code requires a considerable effort, it would also be very hard to keep in secret. The news of their platform being fully or partially re-used at competitor's would have quickly reached GC and they could have shut down the competitor entirely whilst suing them for all of the profit.
Of course one could steal the code for analysis so they could try and exploit the technological weaknesses in GC trading patterns, however this is not the argument given by the article or GC.