A good starting point for Ganassi-related research is Venice’s Archivio di Stato, under the Notarile, Testamenti section (nos. 124–128): that’s where sixteenth-century Venetian notary Francesco Bianco’s registers are located. Among Bianco’s clients were a number of musicians and their friends, including Silvestro Ganassi’s father, a barber, whose will has been preserved.
See Bonnie J. Blackburn: ‘Il magnifico Sigismondo Maler Thedescho’ and his Family: the Venetian connection, in “THE LUTE. The Journal of The Lute Society” Volume L 2010, pp. 60–86.
According to Stefano Pio (Viol and Lute makers of Venice 1490–1630, p. 89, footnote 131), Ganassi's “year of death  is reported on: (ASV) [Archivio di Stato], (SST) [Scuola di San Teodoro, Venice], Mariegola reg. n. 1 c.n.n. For information concerning the financial position of Ganassi see: ASV, Dieci savi alle X, Registro 1471 c. 470 and condizione nuova n. 769 (Ganassi Silvestro e Giacomo).”