Actually,the people at PSE Enterprise are the same people (PhD students, post doc and professors) at the Process System Engineering Centre of the Chemical Engineering Department at Imperial College.
The PhD students get their PhD by writing a gPROM module, plus other academic stuff and doing industrial research on Process Modelling and Control. I am studying my undergraduate degree now at Imperial College.
The original gPROMs was written to improve the functions and add more features to Aspen 1.0. gPROMS then was a pet project of one of the Professors of Chemical Engineering at Imperial College. When Aspen 2.0 was released, the gPROMs code was ported over to support Aspen 2.0 and furthur more features were added. By the time Aspen v4.0 was released, gPROMs was powerful enough that it does not need a new version of Aspen to better it. Instead, Aspen 2.0 was acquired by the university (Imperial College) so that the university has a full ownership to entire gPROMs package, ie. gPROMs module and the Aspen 2.0 core.