"Momadonna cometessa" donated property to the monastery of Santa María de Lara, confirmed by her sons "Fredinando Gundisalviz, Ramiro Gundisalviz", by charter dated 28 Jan 929. His parentage is confirmed by the charter of his grandson Sancho García Conde de Castilla in which the latter names his paternal ancestors as far back as Munio Núñez & his wife Argila. A legend of his kidnap as a young child is commemorated in the 13th century "Poema de Fernán González".
Conde de Castilla. “Moma Donna cometissa...cum filiis meis, quorum nomina subter exarata fiunt” donated property to the monastery of San Millán de Belbimbre by charter dated 1 Feb 929 (the dating clause stating “regnante...comite Fredinando Gundesalviz in Castella”), witnessed by “Fredinando Gundesalviz, Ramiro Gundesalviz...”. "Comes Ferdinandus Gundisalui et uxor mea Sancia" donated property to the monastery of San Quirice de los Ausines by charter dated 30 Apr 929 [must be misdated considering the date of death of Sancha´s second husband]. A charter dated 1 May 932 records the judgment in a dispute involving the monastery of Cardeñas, in the presence of “comite Fredinando Gundissalbiz et suo alfieriz Gomiz Didaz et de Nunu Fernandez et de Munio Gundisalbiz et de Fredinando Didaz et de Albaro Munnioz”, the dating clause of which names "…comite Fredinando Gundisalviz in Castella". “Mumadomna cometissa...cum filio meo Fredinando Gundissalbiz...comes et uxor sua domna Sancia seu neptis meis Gundissalbo Fernandiz vel Sanzio Fernandiz” donated “locum...Balzaramio...” to San Pedro de Cardeñas by charter dated 5 Aug 935, witnessed by "Abolmondar Sarraziniz, Munnio Bellitiz, Muza iben Galeph, Beila Telliz...". During his rule, he united the territory of Castile into a single county, incorporating in particular Lantarón in 935, Cerezo and Grañón in 936. The dating clause of a charter dated 18 Feb 935, under which “Gutier” sold “vinea in Liciniana” to the bishop of Valpuesta, records “regnante domino Ranemiri et comite Fredenando Gundesalbiz in Lantarone”.
Following the humiliating peace imposed on the kingdom of Navarre by Abd al-Rahman III Caliph of Córdoba in 934, the Caliph invaded Castile, sacking Burgos and destroying the monastery of San Pedro de Cardeña before moving into the territory of León. However, Conde Fernando allied himself with Ramiro II King of León and Queen Toda regent of Navarre, their combined forces defeating the Caliph's troops at Alhandega/al-Khandaq, near Simancas in 939. "Fredinando Gondissalvez tocius Castelle comes" donated property to the monastery of San Millán de la Cogolla by charter dated 938, confirmed by "Gondissalvo Fredinandez, Sancio Fredinandez…". Conde Fernando exploited this victory by recapturing Sepúlveda in 940. “Comes Fredinandus Gundisalviz cum uxore mea Sanzia comitissa et cum filiis meis Gundisalvus Fredinandi et Garsea Fredinandi et Sancio Fredinandi et Munio Fredinandi et domna Fronilde” donated the monastery of San Miguel de Javilla to San Pedro de Cardeñas by charter dated 941, witnessed by “Sarracenus, Bermudo Fannez, Gundisalvo de Aza...”. The Cronica de Sampiro records that "Fredenandus Gundissalui et Didacus Munionis" rebelled against King Ramiro II, who captured them and imprisoned them "unum in Legione, alterum in Gordone", dated to [943/44] by Torres, although their imprisonment was short-lived. "Fredinando comes…cum uxore mea…Sancia cometissa" donated property to the monastery of San Millán de la Cogolla by charter dated 944, confirmed by "…Gundissalvo Fernandez, Sancio Fernandez, Garcia Fernandez…". "Ranimiro rex" donated property to Sahagún by charter dated 3 Apr 945, witnessed by "Fredenandus Gundesalviz…". "Ranimiro rex" donated property to Sahagún by charter dated 3 Apr 945, witnessed by "…Fernando Gudisalviz, Gundisalvo Fredenandi…". “...Ansur Fredenandiz, Fredenando Gontesalviz...” confirmed the charter dated 24 Sep 946 under which King Ramiro II donated "villa...Perales et Volpiare et Villa Rezmiro secundum obtinuit illas Gundisalbo Moniz" to Vega. The Cronica de Sampiro records that "avunculo suo…Garsiano rege Pampilonensium, necnon Fredenandus Gundissalui Burgensium comes" helped Sancho in his rebellion against Ordoño III King of León.
Conde in Álava: the dating clause of a charter dated to , under which “Tellu presbiter” donated half his property to Valpuesta, records “regnante...comite Fredenando Gondesalbiz in Alaba et in Castella”. Caliph Abd al-Rahman's troops launched further raids against Galicia and Castile between 953 and 955, but were defeated by Conde Fernando near San Esteban de Gormaz in 955. Conde Fernando became embroiled in the civil war in León, supporting in turn King Sancho I and later his son-in-law King Ordoño IV, but was captured by troops from Navarre at Nájera in 960. After being released by Urraca de Navarra, whom he married, he formed an alliance with Navarre and Barcelona against Caliph Al-Hakam II. The alliance was unsuccessful and Conde Fernando lost San Esteban de Gormaz and Atienza to the Caliph's troops. Although Castile was considerably weakened by Muslim attacks during the last years of his rule, Conde Fernando had been a dynamic leader who established the full autonomy of the county of Castile, and competently defended it against Muslim incursions. His exploits are commemorated in vernacular Castilian romances. "Fredinando Gundissalviz comite…cum filio eius Munio Fredinandiz" confirmed a donation of property at Montorio to the monastery of San Adrian y San Miguel Arcangel by charter dated 6 May 968. .
The Chronicon Burgense records the death in 970 of “comes Fernandus Gundisalvi”. 1