The gross system product, or GSP, was presumably the post-diaspora equivalent of either the gross national product (GNP), or the gross domestic product (GDP). This would define it as the cumulative value added over a T-year by all the labour performed either in the territory of a given star system (GDP equivalent), or by its citizens regardless of where the work was actually performed (GNP equivalent).
In 1900 PD, the Solarian League had, by far, the largest overall production of any star nation (mostly through sheer size, coupled with its core being composed of some of the galaxy's oldest, richest, most populous human colony worlds) and it is implied the Sol System (Earth's solar system) had the highest GSP in the human-settled universe. The Star Kingdom of Manticore's GSP that year, at seventy-eight percent of the GSP of the Sol System, was likely considered very rich by galactic standards. (HH1)
In 1903 PD, Masada had a very poor economy; when the People's Republic of Haven gave Masada two warships (a Sultan-class battlecruiser and a Bastogne-class destroyer), they came to represent over eighty percent of the Endicott system's (i.e., the Masadan star nation's) GSP. Assuming a GSP definition analogous to GNP or GDP, as described above, the vessels themselves, being gifts from another system - not to mention a foreign power - would not count toward the system's GSP. Rather, their value was presumably the same as over four-fifths of the system's actual annual production as measured by GSP. (HH2)