He was born at Urbignacco, a hamlet near Buia (Udine), in June 1703. After entering the ecclesiastic career, since the beginning of the Forties he had been for at least a decade a confessor in the female monastery of Aquileia that had its summer residence at Cividale. At the beginning of the Sixties he was elected a canon of the collegiate church of Cividale where he stayed until his death, in 1779. In the long period he spent at Cividale with great passion and profit Guerra dedicated himself to the research of documents and historical witnesses on his land that he could find in libraries, archives and private collections. The result of this long and fatiguing work were written in the sixty volumes of his Otium Foroiuliense, which collect transcriptions of public and private documents of the patriarchal and the Venetian period of the ‘Patria del Friuli’, as well as other different materials still pertaining to Friulian history, with particular reference to Aquileia and Cividale and to Friulian families and culture. This imposing corpus of documents is sometimes enriched with annotations and observations made by Guerra himself. The handwritten volumes were first purchased by the canon of Cividale Niccolò Portis and later by the Library of the Archaeological Museum of Cividale where they are still kept.
For further information see the entry Guerra Giandomenico, storico written by Lorenzo Di Lenardo in Nuovo Liruti. Dizionario biografico dei Friulani, 2, L’Età Veneta, edited by C. Scalon, C. Griggio, U. Rozzo, Udine, Forum, 2009, 1400-1401.