Giannettini participated to the newspapers Il Roma and Il Secolo d'Italia, as well as to L'Italiano, headed by Pino Romualdi (MSI). In 1963-64, he started to write in the Rivista Militare, a journal of the Italian military general staff, and participated to various NATO meetings.
Giannettini participated to a colloquium on "revolutionary warfare" on 3–5 May 1965 in Rome (Parco dei Principi hotel), which was organized by the Institute Alberto-Pollio, "quasi-exclusively financed by the SIFAR"  military intelligence agency. Giannettini presented there one of the main reports. According to René Monzat, "this colloquium provided the theoretical framework for the strategy of tension." 
Following this colloquium, to which about 20 students participated (among them, Stefano Delle Chiaie and Mario Michele Merlino ), Giannettini and other intervenants were hired by the Italian secret services. In April 1968, these students participated to a trip to Greece of 60 students from the "League of Greek Fascist Students in Italy" and of 51 Italian neo-fascist students, organized by the Greek junta. According to Frédéric Laurent, author of L'Orchestre noir (p. 75), "more than half of the Italians (...) returned from Athens suddenly converted to Anarchism, Leftism, or to Communism, preferably Chinese".
In 1966, Giannettini published with Pino Rauti "Red Hands on the Armed Forces" (Le mani rosse sulle forze armate). In 1969, he participated to an Italian military delegation to West Germany to prepare the buying of Leopard tanks. According to the Swiss brochure made by friends of Giannettini, he was an important agent of the SIFAR and then of the SID ("Agent Z").
During the Parliamentary Commission on Terrorism headed by Senator Giovanni Pellegrino, the Italian Minister of Defense officially recognized before the parliamentaries that Giannettini had been on the Italian secret services' pay-roll. Giorgio Freda, who was acquitted in the trial concerning the 1969 bombings, gave public marks of trust to Giannettini.
According to the magistrate Guido Salvini, in charge of the investigations concerning the 1969 Piazza Fontana bombing, "Guido Giannettini had contacts with Yves Guérin-Sérac in Portugal ever since 1964" 
- René Monzat, Enquêtes sur la droite extrême, Le Monde-éditions, 1992, p.91. Monzat quotes François Duprat, L’Ascension du MSI, Edition les Sept Couleurs, Paris, 1972
- According to a brochure edited in Switzerland, Giorgio Freda « nazi maoïste » ou révolutionnaire inclassable ? followed by Rapport Giannettini sur la stratégie de la tension et de quatre textes sur la révolution européenne, edited by the « Comité de solidarité pour Giorgio Freda » in Lausanne, supplement to Avant-Garde, n°8, imprimerie du Nouvel Ordre social à Genève, 60 pages quoted by René Monzat, 1992, p.91
- Monzat, 1992, p.91
- Monzat, 1992, p.92
- Quoted by Monzat, 1992, p.92
- Monzat, 1992, p.93
- Quoted by Monzat, p.93
- Judge Guido Salvini hearing before the Italian Parliamentary Commission of investigation on terrorism in Italy, 9th session of 12 February 1997 (9ª SEDUTA - MERCOLEDI 12 FEBBRAIO 1997, Presidenza del Presidente PELLEGRINO (Italian), quoted by Daniele Ganser, NATO's Secret Armies: Operation Gladio and Terrorism in Western Europe, ISBN 0-7146-8500-3, 2005, p.120)