Conti was born in Arpino. After studying in Naples with Domenico Gizzi, whom he would later be dubbed after, he made his debut in Rome at an early age, in Leonardo Vinci's Artaserse. According to Carlo Schmidl (1859-1943) and most of the subsequent writers, Gizziello’s debut took place, when he was still aged less than sixteen, in 1730, at the premiere of Vinci’s opera. In fact, his name does not appear in the cast list of the original libretto and it seems exceedingly unlikely that the creation of the title role in a primary author’s opera was entrusted to an inexperienced teenager. His debut should probably be placed around 1733 on the occasion of one of the successful revivals of the opera at Teatro Tordinona. A colourful anecdote relates how another overweening castrato star, Caffarelli, rode post-haste to Rome from Naples just to hear him sing in Artaserse and full of enthusiasm yelled at him: "Bravo, bravissimo Gizziello, it’s Caffariello who's telling you!"
His career led him throughout Europe and especially to London, where he was engaged by George Frideric Handel, with whom he would build up profitable collaboration relations. Conti performed in many of his works, such as Atalanta, Giustino, Berenice and Arminio, as well as in a revival of Ariodante.
He executed later many first performances for the best famed musicians of his time, including Niccolò Jommelli (Manlio, 1746), Baldassare Galuppi (Artaserse, 1751) and Johann Adolf Hasse (Demetrio, 1747).
After 1759 Conti left the stage and settled in Rome, where he spent the last two years of his life.
Being a very sharp soprano for his time (Handel got him repeatedly to reach up to C6), Conti was not quite well disposed towards abuse of coloratura and he chose rather to turn to better account his fluent and smooth style of rendering and expression: he has thus remained famous as a sentimental and gentle singer, but he also always kept, of course, a condition of absolute excellence at vocal virtuosity, even though not so acrobatic as, for instance, that of his contemporary (and friend) Farinelli.
- Dizionario Universale dei Musicisti, Milano, Sonzogno, 1937, ad nomen, as well as, for instance, Dean and Caruselli.
- Kurt Sven Markstrom, The Operas of Leonardo Vinci, Napoletano, Hillsdale NY, Pendragon Press, 2007, p. 304, ISBN 978-1-57647-094-7.
- Almachilde Pellegrini, Spettacoli Lucchesi nei secoli XVII-XIX, in Memorie e Documenti per servire alla storia di Lucca, Tomo XIV, Parte I, Lucca, Giusti, 1914, p. 478 (accessible for free online in Internet Archive). Pellegrini states "late 1732", but the new Tordinona theatre was inaugurated but on 2 January 1733. About the performance history of Artaserse, see also Calvin Wells, Vinci's L'Artaserse (Musikwerkstatt) , "Opera Britannia", 19 February 2010.
- Pellegrini and Markstrom, cited above.
- Caruselli, I, p. 295. Conti always remained in good terms with Farinelli, who repeatedly invited him to Spain, terming him "Antiguo amigo" (longtime friend) (Sandro Cappelletto, La voce perduta: vita di Farinelli, evirato cantore, Turin, EDT, 1995, p. 108. ISBN 88-7063-223-7)
- (Italian) Rodolfo Celletti, Storia del belcanto, Discanto Edizioni, Fiesole, 1983, pp. 105–106, 125.
- (Italian) Salvatore Caruselli (ed.), Grande enciclopedia della musica lirica, Longanesi &C. Periodici S.p.A., Roma, ad nomen.
- Winton Dean, Conti, Gioacchino ['Egizziello', 'Gizziello'], in Stanely Sadie (ed.), The New Grove Dictionary of Opera, Grove (Oxford University Press), New York, 1997, I, pp. 927–928, ISBN 978-0-19-522186-2