Gillespie was described by Joan Copjec as "one of the most gifted and promising philosophers of his generation".
He was a co-founder of the academic journal Umbr(a),.
Gillespie's book The Mathematics of Novelty was published posthumously in 2008.
Peter Hallward wrote that "This tremendously valuable book is a landmark in the critical reception of Badiou’s work."After his death by suicide at age 33, The Mathematics of Novelty was submitted for the degree of Doctor of Philosophy at the University of Warwick, which Gillespie was posthumously awarded in
2005.
Into the first decade of the 21st century, it is now recognized that Gillespie's writings and translations were crucial to the initial reception of Alain Badiou's work in the English-speaking world.